As filed with the Securities and Exchange Commission on March 29, 2018

Registration No. 333-                

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-3

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

Merck & Co., Inc.

(Exact name of registrant as specified in its charter)

2000 Galloping Hill Road

Kenilworth, New Jersey 07033

(908) 740-4000

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Geralyn S. Ritter, Esq.

Senior Vice President and Corporate Secretary

Merck & Co., Inc.

2000 Galloping Hill Road

Kenilworth, New Jersey 07033

(908) 740-4000

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copy to:

Stuart Gelfond, Esq.

Fried, Frank, Harris, Shriver & Jacobson LLP

One New York Plaza

New York, New York 10004

(212) 859-8000

Approximate date of commencement of proposed sale to the public: From time to time after the effective date of this Registration Statement.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box:  ☐

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box:  ☒

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ☐

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.  ☒

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.  ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of Securities Act.  ☐

CALCULATION OF REGISTRATION FEE

PROSPECTUS

Merck & Co., Inc.

Debt Securities

Merck & Co., Inc. may from time to time issue debt securities in one or more offerings pursuant to this prospectus. The accompanying prospectus supplement will specify the terms of the debt securities, including whether they are guaranteed. We urge you to read carefully this prospectus, any accompanying prospectus supplement, and any documents we incorporate by reference in this prospectus and any accompanying prospectus supplement before you make your investment decision.

Merck & Co., Inc. may sell these debt securities to or through underwriters, dealers and agents, or directly to purchasers, on a delayed or continuous basis.

This prospectus describes some of the general terms that may apply to the debt securities and the general manner in which they may be offered. The specific terms of the debt securities, including whether such debt securities will be guaranteed, and the specific manner in which they may be offered, including the names of any underwriters or agents, will be described in a supplement to this prospectus.

Investing in our debt securities involves risks. You should carefully consider all of the information set forth in this prospectus, including under “Risk Factors” on page 2 of this prospectus, and in any prospectus supplement or documents we incorporate by reference in this prospectus or any prospectus supplement before deciding to invest in any of our debt securities.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus. Any representation to the contrary is a criminal offense.

The date of this prospectus is March 29, 2018.

TABLE OF CONTENTS

ABOUT THIS PROSPECTUS

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission, or the Commission, as a “well-known seasoned issuer” as defined in Rule 405 under the Securities Act of 1933, as amended, or the Securities Act, utilizing a “shelf” registration process. Under this shelf process, we may, from time to time, sell any combination of the debt securities described in this prospectus in one or more offerings. No limit exists on the aggregate amount of debt securities we may sell pursuant to the registration statement.

This prospectus provides you with a general description of the debt securities we may offer. Certain portions of the registration statement and the exhibits thereto have been omitted pursuant to the rules and regulations of the Commission. Reference is hereby made to such omitted portions for further information with respect to us and the debt securities offered hereby. A copy of the registration statement may be inspected or obtained from us at the telephone number and address set forth in “Incorporation of Certain Documents by Reference” below. Each time we sell debt securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering, including the specific amounts, prices and terms of the debt securities offered, and whether they will be guaranteed by any of our subsidiaries. Any prospectus supplement may also add, update or change information contained in this prospectus. You should read this prospectus, any prospectus supplement to this prospectus, any documents that we incorporate by reference in this prospectus and any prospectus supplement and the additional information described below under “Where You Can Find More Information” and “Incorporation of Certain Documents by Reference” before making an investment decision. You should rely only on the information contained or incorporated by reference in this prospectus and any prospectus supplement, which may add, update or change information contained in this prospectus. We have not authorized any other person to provide you with additional or different information. If anyone provides you with additional, different or inconsistent information, you should not rely on it. We are not making an offer to sell these securities in any jurisdiction where the offer or sale is not permitted.

Neither the delivery of this prospectus nor any sales hereunder shall under any circumstances create any implication that there has been no change in the affairs of Merck since the date hereof. You should not assume that the information in this prospectus, any accompanying prospectus supplement or any documents we incorporate by reference in this prospectus and any prospectus supplement is accurate as of any date other than the date on the front of those documents or on other dates which are specified in those documents. Our business, financial condition, results of operations and prospects may have changed since that date.

1

MERCK

We are a global health care company that delivers innovative health solutions through our prescription medicines, vaccines, biologic therapies and animal health products. Our operations are principally managed on a products basis and include four operating segments, which are the Pharmaceutical, Animal Health, Healthcare Services and Alliances segments. The Pharmaceutical segment is the only reportable segment.

The Pharmaceutical segment includes human health pharmaceutical and vaccine products. Human health pharmaceutical products consist of therapeutic and preventive agents, generally sold by prescription, for the treatment of human disorders. We sell these human health pharmaceutical products primarily to drug wholesalers and retailers, hospitals, government agencies and managed health care providers such as health maintenance organizations, pharmacy benefit managers and other institutions. Vaccine products consist of preventive pediatric, adolescent and adult vaccines, primarily administered at physician offices. We sell these human health vaccines primarily to physicians, wholesalers, physician distributors and government entities.

We also have an Animal Health segment that discovers, develops, manufactures and markets animal health products, including vaccines, which we sell to veterinarians, distributors and animal producers. Our Healthcare Services segment provides services and solutions that focus on engagement, health analytics and clinical services to improve the value of care delivered to patients.

We were incorporated in the State of New Jersey and maintain our principal offices in Kenilworth, New Jersey. Our address is 2000 Galloping Hill Road, Kenilworth, New Jersey 07033, and our telephone number is (908) 740-4000. Our website is located at www.merck.com. Information available on, or accessible through, our website is not incorporated into this prospectus by reference and should not be considered a part of this prospectus.

RISK FACTORS

Before deciding to invest in Merck’s debt securities, you should carefully consider the risk factors and forward-looking statements described in Item 1A of our most recent Annual Report on Form 10-K for the year ended December 31, 2017 (which is incorporated by reference herein). In addition, you should carefully consider information in any accompanying prospectus supplement or any documents we incorporate by reference in this prospectus and any accompanying prospectus supplement, before deciding to invest in Merck’s debt securities. Additional risks and uncertainties not presently known to us or that we currently deem immaterial may also impair our business operations.

FORWARD-LOOKING STATEMENTS

This prospectus, any prospectus supplement and any documents we incorporate by reference herein or therein and oral statements made from time to time by us may contain so called “forward-looking statements” (within the meaning of Section 27A of the Securities Act, and Section 21E of the Securities Exchange Act of 1934, as amended, or the Exchange Act), all of which are based on management’s current expectations and are subject to risks and uncertainties which may cause results to differ materially from those set forth in the statements. One can identify these forward-looking statements by their use of words such as “anticipates,” “expects,” “plans,” “will,” “estimates,” “forecasts,” “projects” and other words of similar meaning, or negative variations of any of the foregoing. One can also identify them by the fact that they do not relate strictly to historical or current facts. These statements are likely to address our growth strategy, financial results, product development, product approvals, product potential and development programs. One must carefully consider any such statement and should understand that many factors could cause actual results to differ materially from our forward-looking statements. These factors include inaccurate assumptions and a broad variety of other risks and uncertainties,

2

including some that are known and some that are not known. No forward-looking statement can be guaranteed and actual future results may vary materially. We do not assume the obligation to update any forward-looking statement. We caution you not to place undue reliance on these forward-looking lstatements. One should carefully evaluate such statements in light of factors, including risk factors, described under “Risk Factors” above and in the documents incorporated herein by reference in which we discuss in more detail various important factors that could cause actual results to differ from expected or historic results. One should understand that it is not possible to predict or identify all such factors. Consequently, one should not consider any such list to be a complete statement of all potential risks or uncertainties.

RATIOS OF EARNINGS TO FIXED CHARGES

Our consolidated ratios of earnings to fixed charges for each of the fiscal years ended December 31, 2013 through 2017 are as follows:

For purposes of computing these ratios, “earnings” consist of income before taxes, one-third of rents (deemed by us to be representative of the interest factor inherent in rents), interest expense, interest capitalized, net of amortization and equity (income) loss from affiliates, net of distributions. “Fixed charges” consist of one-third of rents, interest expense and dividends on preferred stock. Interest expense does not include interest related to uncertain tax positions.

USE OF PROCEEDS

Unless otherwise indicated in the accompanying prospectus supplement, we will use the net proceeds from the sale of the debt securities for general corporate purposes, including the reduction of short-term debt. We may temporarily invest funds that we do not immediately need for these purposes in short-term marketable securities.

3

DESCRIPTION OF DEBT SECURITIES WE MAY OFFER

General

In this description “we,” “our” and “us” refer to Merck & Co., Inc., but not to any of Merck & Co., Inc.’s subsidiaries. “You” means direct holders and not street name or other indirect holders of debt securities. Indirect holders should read the information under the caption “Legal Ownership and Book-Entry Issuance.”

The debt securities are not secured by any of our property or assets. Accordingly, your ownership of debt securities means you are one of our unsecured creditors. The debt securities are not subordinated to any of our other debt obligations and therefore they rank equally with all our other unsecured and unsubordinated indebtedness.

As required by federal law for all bonds and notes that are publicly offered, a document called the indenture governs the debt securities. The indenture is a contract, dated as of January 6, 2010, which we may amend in the future, between us and U.S. Bank Trust National Association, which acts as trustee. The trustee has two main roles. First, the trustee can enforce your rights against us if we default. There are some limitations on the extent to which the trustee acts on your behalf, described under “—Defaults and Remedies—Events of Default—Remedies if an Event of Default Occurs.” Second, the trustee performs administrative duties for us, such as sending you interest payments, registering transfers of your debt securities to a new buyer if you sell and sending you notices.

The indenture and its associated documents contain the full legal text of the matters described in this section. New York law governs the indenture and will govern the debt securities. The indenture is an exhibit to our registration statement of which this prospectus forms a part. See “Where You Can Find More Information” for information on how to obtain a copy.

We may issue as many distinct series of debt securities under the indenture as we wish. A series of debt securities may be guaranteed by one or more of our subsidiaries. There is no limit on the amount of debt securities we may issue under the indenture and the provisions of the indenture allow us to issue debt securities with terms different from those previously issued under the indenture. Also, we may “reopen” a previous issue of a series of debt securities and issue additional debt securities of that series. We may issue debt securities in amounts that exceed the total amount specified on the cover of your prospectus supplement at any time without your consent and without notifying you.

This section summarizes all the material terms of the debt securities that are common to all series unless otherwise indicated in the prospectus supplement relating to a particular series. Because this section is a summary, it does not describe every aspect of the debt securities and is subject to and qualified in its entirety by reference to all the provisions of the indenture, including definitions of some of the terms used in the indenture. We describe the meaning for only some of the important terms. We also include references in parentheses to some sections of the indenture. Whenever we refer to particular sections or defined terms of the indenture in this prospectus or in the prospectus supplement, we incorporate by reference those sections or defined terms here or in the prospectus supplement.

We may issue the debt securities as original issue discount securities, which we would offer and sell at a substantial discount below their stated principal amount. (section 101) A prospectus supplement relating to original issue discount securities will describe federal income tax consequences and other special considerations applicable to them. We may also issue the debt securities as indexed securities or securities denominated in foreign currencies, currency units or composite currencies, as described in more detail in a prospectus supplement relating to any of these types of debt securities. A prospectus supplement relating to indexed debt securities or foreign currency debt securities will also describe any additional tax consequences or other special considerations applicable to these types of debt securities.

4

In addition, we will describe the material specific financial, legal and other terms particular to debt securities of each series in a prospectus supplement relating to debt securities of that series. A prospectus supplement relating to debt securities of a series will describe the following terms of the debt securities:

5

We will attach the prospectus supplement relating to the debt securities of the series to the front of this prospectus.

We may issue debt securities other than the debt securities described in this prospectus. There is no requirement that we issue any other debt securities under the indenture described herein. Thus, we may issue any other debt securities under other indentures or documentation, containing provisions different from those included in the indenture or applicable to one or more issues of the debt securities described in this prospectus.

Without limiting the foregoing, we and the trustee may enter into one or more supplemental indentures with one or more of our subsidiaries providing for a full and unconditional guarantee by such subsidiaries of the payment of principal of, premium, if any, and interest on and “additional amounts” with respect to these debt securities when due, whether at maturity or otherwise. The debt securities described herein will be effectively subordinated to any secured debt we or our subsidiaries incur to the extent of the value of such security. The debt securities will also be structurally subordinated to all indebtedness of our subsidiaries which are not guarantors of the debt securities.

Overview of Remainder of this Description

The remainder of this description summarizes:

Additional Mechanics

Form, Exchange and Registration of Transfer

We will issue the debt securities:

You may have your debt securities broken into more debt securities of smaller denominations of not less than $2,000 or combined into fewer debt securities of larger denominations, as long as the total principal amount is not changed. (section 305) This is called an exchange.

You may exchange or register a transfer of debt securities at the office of the trustee. The trustee acts as our agent for registering debt securities in the names of holders and registering transfers of debt securities. We may change

6

this appointment to another entity or perform it ourselves. The entity performing the role of maintaining the list of registered direct holders is called the security registrar. It will also register transfers. (section 305) You may also replace lost, stolen or mutilated debt securities at that office. (section 306) The trustee’s agent may require an indemnity before replacing any debt securities.

You will not be required to pay a service charge to register a transfer of debt securities or to exchange debt securities, but you may be required to pay for any tax or other governmental charge associated with the transfer or exchange. The security registrar will make the registration of transfer or exchange only if it is satisfied with your proof of ownership. (section 305)

If we have designated additional trustees, they are named in the prospectus supplement. We may cancel the designation of any particular trustee. We may also approve a change in the office through which any trustee acts. (section 1002)

If the debt securities are redeemable and we redeem less than all of the debt securities of a particular series, we may block the issuance of, registration of transfer or exchange of debt securities during the period beginning 15 days before the day we mail the notice of redemption and ending on the day of that mailing, in order to freeze the list of holders to prepare the mailing. We may also refuse to register transfers or exchanges of debt securities selected for redemption, except that we will continue to permit transfers and exchanges of the unredeemed portion of any debt security being partially redeemed. (section 305)

The rules for exchange described above apply to exchange of debt securities for other debt securities of the same series and tenor.

Payment and Paying Agents

We will pay interest to you on each date interest is due if you are a direct holder listed in the trustee’s records at the close of business on a particular day in advance of each due date for interest, even if you no longer own the debt security on the interest due date. That particular day is called the regular record date and is stated in the prospectus supplement. (section 307) Holders buying and selling debt securities must work out between them how to compensate for the fact that we will pay all the interest for an interest period to the one who is the registered holder on the regular record date.

Unless otherwise stated in the prospectus supplement, we will pay interest, principal and any other money due on the debt securities at the corporate trust office of the trustee in New York City. (section 1002) That office is currently located at 100 Wall Street, 16th floor, New York, New York 10005. You must make arrangements to have your payments picked up at or wired from that office. We may also choose to pay interest by mailing checks.

Street name and other indirect holders should consult their banks or brokers for information on how they will receive payments.

We may also arrange for additional payment offices, and may cancel or change these offices, including our use of the trustee’s corporate trust office. These offices are called paying agents. We may also choose to act as our own paying agent. We must notify you of changes in the paying agents for any particular debt securities of the series. (section 1002)

Notices

We and the trustee will send notices regarding the debt securities only to direct holders, using their addresses as listed in the trustee’s records. (section 106)

7

All paying agents must return to us upon our request all money paid by us that remains unclaimed two years after the amount is due to direct holders. After that two-year period, you may look only to us for payment and not to the trustee, any other paying agent or anyone else. (section 1003)

Special Situations

Mergers and Similar Events

We may consolidate or merge with another company or firm. We may also convey, transfer or lease all of our properties and assets substantially as an entirety to another firm, or buy or lease substantially all of the assets of another firm. However, we may not take any of these actions unless the following conditions, among others, are met:

If the conditions described above are satisfied with respect to any series of debt securities, we will not need to obtain the approval of the holders of those debt securities in order to merge or consolidate or to sell our assets. Also, these conditions will apply only if we wish to merge or consolidate with another entity or convey, transfer or lease all of our properties and assets substantially as an entirety. We will not need to satisfy these conditions if we enter into other types of transactions, including any transaction in which we acquire the stock or assets of another entity, any transaction that involves a change of control but in which we do not merge or consolidate and any transaction in which we convey, transfer or lease less than all of our properties and assets substantially as an entirety. It is possible that these other types of transactions may result in a reduction in our credit rating, may reduce our operating results or may impair our financial condition. However, you will have no approval right with respect to any transaction of this type.

Modification and Waiver

There are three types of changes we can make to the indenture and the debt securities.

Changes Requiring Your Approval. First, there are changes that cannot be made to your debt securities without your specific approval. Following is a list of those types of changes:

8

Changes Requiring a Majority Vote. The second type of change to the indenture and the debt securities is the kind that requires a vote in favor by direct holders owning not less than a majority of the principal amount of the debt securities of the particular series affected. (section 902) Most changes fall into this category, such as if we wish to obtain a waiver of all or part of the restrictive covenants described below, or a waiver of a past default. However, we cannot obtain a waiver of a payment default or any other aspect of the indenture or the debt securities listed in the first category above under “—Changes Requiring Your Approval” unless we obtain your individual consent to the waiver. (section 513)

Changes Not Requiring Approval. The third type of change does not require any vote by holders of debt securities. This type is limited to the addition or release of a guarantee, corrections and clarifications and other changes that would not adversely affect holders of the debt securities. (section 901)

Further Details Concerning Voting. When taking a vote, we will use the following rules to decide how much principal amount to attribute to a debt security:

Debt securities will not be considered outstanding and therefore will not carry voting rights if we have deposited or set aside in trust for you money for their payment or redemption. Debt securities will also not be eligible to vote if they have been fully defeased as described under “—Defeasance—Full Defeasance.” (section 1302)

We may set any day as a record date for the purpose of determining the direct holders of outstanding debt securities that are entitled to vote or take other action under the indenture. (section 104) In some circumstances, the trustee may set a record date for action by direct holders.

Street name and other indirect holders should consult their banks or brokers for information on how approval may be granted or denied if we seek to change the indenture or the debt securities or request a waiver.

9

Restrictive Covenants

In the following description of restrictive covenants, we use several specialized terms without explaining the meaning when we use the terms. We define these terms, which appear in bold, italicized type without quotation marks the first time they appear, in “—Definitions Relating to our Restrictive Covenants” at the end of this subsection.

Restrictions on Secured Debt. Some of our property may be subject to a mortgage or other legal mechanism that gives our lenders preferential rights in that property over other lenders, including you and the other direct holders of the debt securities, or over our general creditors if we fail to pay them back. These preferential rights are called liens. Debt which is protected by these preferential rights is called secured debt. In the indenture, we promise that neither we nor our domestic subsidiaries will incur any new secured debt that is secured by a lien on any of our or our domestic subsidiaries’ principal domestic manufacturing properties, or on any shares of stock of any of our domestic subsidiaries that own or lease a principal domestic manufacturing property, unless we grant an equivalent or higher-ranking lien on the same property to you and the other direct holders of the debt securities.

We do not need to comply with this restriction if the amount of all debt that would be secured by liens on principal domestic manufacturing properties, including the new debt, the debt securities which we would so secure as described in the previous sentence, and all attributable debt that results from a sale and leaseback transaction involving principal domestic manufacturing properties, is less than 10% of our consolidated net tangible assets.

This restriction on secured debt does not apply to debt secured by certain types of liens, and we can disregard this secured debt when we calculate the limits imposed by this restriction. These types of liens are:

We and our subsidiaries may have as much unsecured debt as we may choose. (section 1006)

Restrictions on Sales and Leasebacks. We promise that neither we nor any of our domestic subsidiaries will enter into any sale and leaseback transaction involving a principal domestic manufacturing property, unless we comply with this restrictive covenant. A sale and leaseback transaction generally is an arrangement between us or a

10

domestic subsidiary and a bank, insurance company or other lender or investor where we or the domestic subsidiary sell a property to a lender or investor more than 120 days after the acquisition of the property or the completion of construction of the property and the beginning of its full operation and we lease the property back from the lender.

We can comply with this restrictive covenant in either of two ways:

This restriction on sales and leasebacks does not apply to any sale and leaseback transaction that is between us and one of our domestic subsidiaries or between domestic subsidiaries, or that involves a lease for a period of three years or less. (section 1007)

Definitions Relating to our Restrictive Covenants. Following are summaries of the meanings of the terms that are important in understanding the restrictive covenants previously described:

11

Defeasance

The following discussion of full defeasance and covenant defeasance will be applicable to your debt securities only if we choose to have those provisions apply to securities of that series. If we do so choose, we will state that in the prospectus supplement. (section 1301)

Full Defeasance. If there is a change in federal tax law, as described below, we can legally release ourselves from any payment or other obligations on the debt securities of a series if we put in place other arrangements for you to be repaid. This is called full defeasance. In order to achieve full defeasance, we must do the following, among other things:

If we ever did accomplish full defeasance, as described above, you would have to rely solely on the trust deposit for repayment on the debt securities. You could not look to us for repayment in the unlikely event of any shortfall. (sections 1302 and 1304)

Covenant Defeasance. Under current federal tax law, we can make the same type of deposit described above and be released from some of the restrictive covenants in the debt securities. This is called covenant defeasance. In that event, you would lose the protection of those restrictive covenants but would gain the protection of having money and securities set aside in trust to repay the debt securities. In order to achieve covenant defeasance of the debt securities of a series, we must do the following:

If we accomplish covenant defeasance, the following provisions of the indenture and the debt securities would no longer apply:

12

If we accomplish covenant defeasance, you can still look to us for repayment of the debt securities if there were a shortfall in the trust deposit. In fact, if one of the remaining events of default occurred, such as our bankruptcy, and the debt securities become immediately due and payable, there may be such a shortfall in the trust deposit. (sections 1303 and 1304)

Default and Remedies—Events of Default

You will have special rights if an event of default occurs and is not cured, as described later in this subsection.

What Is an Event of Default?

The term event of default with respect to your series of debt securities means any of the following:

Remedies if an Event of Default Occurs.

If an event of default has occurred and has not been cured, the trustee or the direct holders of at least 25% in principal amount of the outstanding debt securities of the affected series may declare the entire principal amount of all the debt securities of that series to be due and immediately payable. This is called a declaration of acceleration. The direct holders of at least a majority in principal amount of the debt securities of the affected series may cancel a declaration of acceleration of maturity. (section 502)

Except in cases of default, where the trustee has some special duties, the trustee is not required to take any action under the indenture at the request of any holders unless the direct holders offer the trustee reasonable protection, called an indemnity, against expenses and liability. (section 603) If reasonable indemnity is provided, the direct holders of a majority in principal amount of the outstanding debt securities of the relevant series may direct the time, method and place of conducting any lawsuit or other formal legal action seeking any remedy available to the trustee. These majority direct holders may also direct the trustee in performing any other action under the indenture. (section 512)

Before you bypass the trustee and bring your own lawsuit or other formal legal action or take other steps to enforce your rights or protect your interests relating to the debt securities, the following must occur:

13

However, you are entitled at any time to bring a lawsuit for the payment of money due on your debt security on or after its due date. (section 508)

Street name and other indirect holders should consult their banks or brokers for information on how to give notice or direction to or make a request of the trustee and to make or cancel a declaration of acceleration.

We will furnish to the trustee every year a written statement of our principal executive, financial or accounting officer certifying that to the best of such signer’s knowledge we are in compliance with the indenture and the debt securities, or else specifying any default. (section 1004)

Our Relationship with the Trustee

U.S. Bank Trust National Association is the trustee under the indenture. The trustee performs services for us in the ordinary course of business.

LEGAL OWNERSHIP AND BOOK-ENTRY ISSUANCE

Street Name and Other Indirect Holders

We generally will not recognize investors who hold debt securities in accounts at banks or brokers as legal holders of debt securities. Holding in that way is called holding in street name. Instead, we would recognize only the bank or broker, or the financial institution the bank or broker uses to hold its debt securities. These intermediary banks, brokers and other financial institutions pass along principal, interest and other payments on the debt securities, either because they agree to do so in their customer agreements or because they are legally required to. If you hold debt securities in street name, you should check with your own institution to find out:

Direct Holders

Our obligations, as well as the obligations of the trustee and those of any third parties employed by us or the trustee, run only to persons or entities who are the direct holders of debt securities, i.e., those who are registered as holders of debt securities. As noted above, we do not have obligations to you if you hold in street name or

14

through other indirect means, either because you choose to hold debt securities in that manner or because we issued the debt securities in the form of global securities as described below. For example, once we make payment to the registered holder, we have no further responsibility for the payment even if that registered holder is legally required to pass the payment along to you as a street name customer but does not do so.

Global Securities

What is a Global Security? A global security is a special type of indirectly held security, as described above under “—Street Name and Other Indirect Holders.” If we choose to issue debt securities in the form of global securities only, the ultimate beneficial owners can only be indirect holders. We do this by requiring that the global security be registered in the name of a financial institution or clearing system, or their nominee, that we select and by requiring that the debt securities included in the global security not be transferred to the name of any other direct holder unless the special circumstances described below occur. The financial institution that acts as the sole direct holder of the global security is called the depository. The Depository Trust Company, New York, New York, known as DTC, may be a depository for one or more series of debt securities. For information regarding DTC, see “—Considerations Relating to DTC.”

Any person wishing to own a debt security included in a global security must do so indirectly by virtue of an account with a broker, bank or other financial institution that in turn has an account with the depository. The prospectus supplement indicates whether we will issue your debt securities only in the form of global securities.

Special Investor Considerations for Global Securities. The account rules of your financial institution and the rules of the depository, as well as general laws relating to securities transfers, will govern your rights as an indirect holder of a global security. We will not recognize you as a registered holder of debt securities and instead will deal only with the depository that holds the global security.

You should be aware that if debt securities are issued only in the form of global securities:

Special Situations When a Global Security will be Terminated. In a few special situations described below, the trustee will terminate the global security and will exchange interests in it for separate certificates representing

15

debt securities. After that exchange, the choice of whether to hold debt securities directly or in street name will be up to you. You must consult your own bank or broker to find out how to have your interests in the debt securities transferred to your own name, so that you will be a direct holder. We previously described the rights of street name investors and direct holders in the debt securities in the subsections entitled “—Street Name and Other Indirect Holders” and “—Direct Holders.”

The special situations for termination of a global security are:

The prospectus supplement may also list additional situations for terminating a global security that would apply only to the particular debt securities of the series covered by the prospectus supplement. When a global security terminates, the depository, and not we or the trustee, is responsible for deciding the names of the institutions that will be the initial direct holders. (sections 204 and 305)

Considerations Relating to DTC. DTC has informed us that it is a limited-purpose trust company organized under the New York Banking Law, a “banking organization” within the meaning of the New York Banking Law, a member of the Federal Reserve System, a “clearing corporation” within the meaning of the New York Uniform Commercial Code and a “clearing agency” registered pursuant to the provisions of Section 17A of the Exchange Act. DTC holds securities that DTC participants deposit with DTC. DTC also facilitates the post-trade settlement among DTC participants of securities transactions, such as transfers and pledges in deposited securities through electronic computerized book-entry changes in DTC participants’ accounts, thereby eliminating the need for physical movement of certificates. DTC participants include securities brokers and dealers, banks, trust companies and clearing corporations, and certain other organizations. Indirect access to the DTC system also is available to others such as banks, brokers, dealers, trust companies and clearing corporations that clear through or maintain a custodial relationship with a participant, either directly or indirectly. DTC is a wholly owned subsidiary of The Depository Trust & Clearing Corporation, or DTCC. DTCC is the holding company for DTC, National Securities Clearing Corporation and Fixed Income Clearing Corporation, all of which are registered clearing agencies. DTCC is owned by the users of its regulated subsidiaries. The rules applicable to DTC and DTC participants are on file with the Commission.

Purchases of securities within the DTC system must be made by or through DTC participants, which will receive a credit for the securities on DTC’s records. The ownership interest of each actual purchaser of the notes, which we refer to as the “beneficial owner,” is in turn to be recorded on the DTC participants’ records. Beneficial owners will not receive written confirmation from DTC of their purchase, but beneficial owners are expected to receive written confirmations providing details of the transactions, as well as periodic statements of their holdings from the direct or indirect DTC participant through which the beneficial owner entered into the transaction. Transfers of ownership interests in the global notes will be effected only through entries made on the books of DTC participants acting on behalf of beneficial owners. Beneficial owners will not receive certificates representing their ownership interests in the global notes, except in the event that use of the book-entry system for the notes is discontinued.

To facilitate subsequent transfers, all global securities deposited by direct participants with DTC will be registered in the name of DTC’s partnership nominee, Cede & Co., or such other name as may be requested by an authorized representative of DTC. The deposit of securities with DTC and their registration in the name of Cede & Co. or such other nominee will not change the beneficial ownership of the securities. DTC has no

16

knowledge of the actual beneficial owners of the securities. DTC’s records reflect only the identity of the direct participants to whose accounts the securities are credited, which may or may not be the beneficial owners. The participants are responsible for keeping account of their holdings on behalf of their customers.

Conveyance of notices and other communications by DTC to direct participants, by direct participants to indirect participants, and by direct Participants and indirect participants to beneficial owners will be governed by arrangements among them, subject to any statutory or regulatory requirements as may be in effect from time to time.

Redemption notices will be sent to DTC. If less than all of the securities of a particular series are being redeemed, DTC’s practice is to determine by lot the amount of the interest of each direct participant in the securities of such series to be redeemed.

In instances in which a vote is required, generally, neither DTC nor Cede & Co. will itself consent or vote with respect to the securities. Under its usual procedures DTC would mail an omnibus proxy to us as soon as possible after the record date. The omnibus proxy assigns Cede & Co.’s consenting or voting rights to those direct participants to whose accounts such securities are credited on the record date (identified in a listing attached to the omnibus proxy).

Redemption proceeds, distributions and payments on the securities will be made by the trustee to Cede & Co., as nominee of DTC. DTC’s practice is to credit direct participants’ accounts upon DTC’s receipt of funds and corresponding detail information from us on the payment date in accordance with their respective holdings shown on DTC records. Payments by participants to beneficial owners will be governed by standing instructions and customary practices and will be the responsibility of such participants and not of DTC, the trustee or us, subject to any statutory or regulatory requirements as may be in effect from time to time. Payment of redemption proceeds, distributions and dividend payments to Cede & Co. (or such other nominee as may be requested by an authorized representative of DTC) is the responsibility of us or the trustee, disbursement of such payments to direct participants will be the responsibility of DTC, and disbursement of such payments to the beneficial owners will be the responsibility of DTC, and disbursements of such payment to the beneficial owners will be the responsibility of direct and indirect participants.

The information in this section concerning DTC and DTC’s book-entry system has been obtained from sources that we believe to be accurate, but we assume no responsibility for the accuracy thereof. We do not have any responsibility for the performance by DTC or its participants of their respective obligations as described herein or under the rules and procedures governing their respective operations.

17

PLAN OF DISTRIBUTION

We may sell debt securities:

The debt securities we distribute by any of these methods may be sold to the public, in one or more transactions, either:

We may sell debt securities from time to time to one or more underwriters, who would purchase the securities as principal for resale to the public, either on a firm-commitment or best-efforts basis. If we sell debt securities to underwriters, we may execute an underwriting agreement with them at the time of sale and will name them in the applicable prospectus supplement. In connection with those sales, underwriters may be deemed to have received compensation from us in the form of underwriting discounts or commissions and may also receive commissions from purchasers of the debt securities for whom they may act as agents. Underwriters may resell the debt securities to or through dealers, and those dealers may receive compensation in the form of discounts, concessions or commissions from the underwriters and/or commissions from purchasers for whom they may act as agents. The prospectus supplement will include any required information about underwriting compensation we pay to underwriters, and any discounts, concessions or commissions underwriters allow to participating dealers, in connection with an offering of debt securities.

We may solicit offers to purchase debt securities directly from the public from time to time. We may also designate agents from time to time to solicit offers to purchase debt securities from the public on our behalf. If required, the prospectus supplement relating to any particular offering of debt securities will name any agents designated to solicit offers, and will include information about any commissions we may pay the agents, in that offering. Agents may be deemed to be “underwriters” as that term is defined in the Securities Act.

From time to time, we may sell debt securities to one or more dealers acting as principals. The dealers, who may be deemed to be “underwriters” as that term is defined in the Securities Act, may then resell those debt securities to the public.

Any underwriter or agent involved in the offer and sale of any debt securities will be named in the prospectus supplement.

Underwriters, agents and dealers may be entitled, under agreements with us, to indemnification against certain civil liabilities, including liabilities under the Securities Act.

Each series of debt securities will be a new issue, and there will be no established trading market for any debt security prior to its original issue date. We may, but are not required to, list a particular series of debt securities on a securities exchange or quotation system. Any underwriters to whom we sell debt securities for public offering may make a market in those debt securities. However, no such underwriter that makes a market will be obligated to do so, and any of them may stop doing so at any time without notice. No assurance can be given as to the liquidity or trading market for any of the debt securities.

18

Unless otherwise indicated in your prospectus supplement or confirmation of sale, the purchase price of the debt securities will be required to be paid in immediately available funds in New York City.

In connection with an offering, the underwriters may purchase and sell debt securities in the open market. These transactions may include short sales, stabilizing transactions and purchases to cover positions created by short sales. Short sales involve the sale by the underwriters of a greater number of debt securities than they are required to purchase in an offering. Stabilizing transactions consist of certain bids or purchases made for the purpose of preventing or retarding a decline in the market price of the debt securities while an offering is in progress.

The underwriters may also impose a penalty bid. This occurs when a particular underwriter repays to the underwriters a portion of the underwriting discount received by it because the underwriters have repurchased debt securities sold by or for the account of that underwriter in stabilizing or short-covering transactions.

These activities by the underwriters may stabilize, maintain or otherwise affect the market price of the debt securities. As a result, the price of the debt securities may be higher than the price that otherwise might exist in the open market. If these activities are commenced, they may be discontinued by the underwriters at any time. These transactions may be effected on an exchange or automated quotation system, if the debt securities are listed on that exchange or admitted for trading on that automated quotation system, or in the over-the-counter market or otherwise.

Underwriters, dealers and agents may engage in transactions with or perform services for us in the ordinary course of their businesses.

19

VALIDITY OF DEBT SECURITIES

Unless otherwise specified in a prospectus supplement, Michael J. Holston, our Executive Vice President and General Counsel, will pass upon the validity of the debt securities for us. As of March 29, 2018, Mr. Holston owned, directly or indirectly, 69,786.227 shares of our common stock, zero shares of restricted stock and options to purchase 344,458 additional shares of our common stock, of which 109,422 were then exercisable.

EXPERTS

The financial statements and management’s assessment of the effectiveness of internal control over financial reporting (which is included in Management’s Report on Internal Control over Financial Reporting) incorporated in this prospectus by reference to the Annual Report on Form 10-K for the year ended December 31, 2017 have been so incorporated in reliance on the report of PricewaterhouseCoopers LLP, an independent registered public accounting firm, given on the authority of said firm as experts in auditing and accounting.

WHERE YOU CAN FIND MORE INFORMATION

We are a reporting company under the Exchange Act and file annual, quarterly and current reports, proxy statements and other information with the Commission. You may read and copy any materials filed with the Commission at the Commission’s Public Reference Room at 100 F Street, N.E., Washington, DC 20549. You may obtain information on the operation of the Public Reference Room by calling the Commission at 1-800-SEC-0330. Also, the Commission maintains a website that contains reports, proxy and information statements, and other information regarding issuers, including us, that file electronically with the Commission. The public can obtain any documents that we file electronically with the Commission at the Commission’s website, http://www.sec.gov, or through the New York Stock Exchange, 20 Broad Street, New York, New York 10005, on which our common stock is listed. In addition, you may request copies of these filings at no cost by writing or telephoning us at the following address: Corporate Secretary, Merck & Co., Inc., 2000 Galloping Hill Road, Kenilworth, NJ 07033, (908) 740-4000; or on our website.

We have filed with the Commission a registration statement on Form S-3 relating to the securities covered by this prospectus. This prospectus is a part of the registration statement and does not contain all the information in the registration statement. Whenever a reference is made in this prospectus or any prospectus supplement to a contract or other document filed or incorporated by reference as an exhibit to our registration statement, the reference is only a summary. For a copy of the contract or other document, you should refer to the exhibits that are a part of the registration statement or incorporated by reference into the registration statement by the filing of a Current Report on Form 8-K or otherwise. You may review a copy of the registration statement and the documents we incorporate by reference at the Commission’s Public Reference Room in Washington, D.C., as well as through the Commission’s website as listed above.

INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

The following documents, filed with the Commission (File No. 001-06571) pursuant to the Exchange Act are incorporated by reference in this prospectus: (a) our Annual Report on Form 10-K for the fiscal year ended December 31, 2017; (b) our Proxy Statement on Schedule 14A filed with the Commission on April 10, 2017, to the extent specifically incorporated by reference into Part III of our Annual Report on Form 10-K for the fiscal year ended December 31, 2016; and (c) our Current Reports on Form 8-K filed on January 26, 2018 and February 26, 2018.

Also, all documents filed by us with the Commission under File No. 001-06571 pursuant to Sections 13(a), 13(c), 14, or 15(d) of the Exchange Act (not including Current Reports or portions thereof furnished under Item 2.02 or

20

Item 7.01 under Form 8-K) after the date of this prospectus and prior to termination of the relevant offering shall be deemed to be incorporated by reference herein and to be a part hereof from the date of such filing. Any statement contained in a document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded for purposes hereof to the extent that a statement contained herein or in any other subsequently filed document that also is, or is deemed to be, incorporated by reference herein modifies or supersedes such statement. Any such statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this prospectus.

We will provide, without charge, copies of any document incorporated by reference into this prospectus, excluding exhibits other than those that are specifically incorporated by reference in this prospectus. You can obtain a copy of any document incorporated by reference by writing or calling us at our principal executive offices as follows:

Merck & Co., Inc.

2000 Galloping Hill Road

Kenilworth, NJ 07033 USA

908-740-1825

Attention: Office of the Secretary

Information on our website is not part of this prospectus, and you should not rely on that information in making your investment decision unless that information is also in this prospectus or has been expressly incorporated by reference into this prospectus.

21

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

The following table sets forth all fees and expenses payable by the Registrant in connection with the issuance and distribution of the debt securities being registered hereby (other than underwriting or broker-dealer discounts and commissions). All of the amounts shown are estimates.

The New Jersey Business Corporation Act provides that a New Jersey corporation has the power to indemnify a director, officer, employee or agent against his or her expenses and liabilities in connection with any proceeding involving the director, officer, employee or agent by reason of his or her being or having been such a director, officer, employee or agent, other than a proceeding by or in the right of the corporation, if such a director, officer, employee or agent acted in good faith and in a manner he or she reasonably believed to be in or not opposed to the best interests of the corporation; and with respect to any criminal proceeding, such director, officer, employee or agent had no reasonable cause to believe his or her conduct was unlawful.

The indemnification and advancement of expenses shall not exclude any other rights, including the right to be indemnified against liabilities and expenses incurred in proceedings by or in the right of the corporation, to which a director or officer may be entitled under a certificate of incorporation, bylaw, agreement, vote of stockholders, or otherwise; provided that no indemnification shall be made to or on behalf of a director or officer if a judgment or other final adjudication adverse to the director or officer establishes that his or her acts or omissions (a) were in breach of his or her duty of loyalty to the corporation or its stockholders, (b) were not in good faith or involved a knowing violation of law or (c) resulted in receipt by the director or officer of an improper personal benefit.

The Restated Certificate of Incorporation of Merck & Co., Inc. (the “Company”) provides that, to the fullest extent permitted by the laws of the State of New Jersey, all current and former directors and officers of the Company shall not be personally liable to the Company or its stockholders for damages for breach of any duty owed to the Company or its stockholders, except that a director or officer shall not be relieved from liability for any breach of duty based upon an act or omission (a) in breach of such person’s duty of loyalty to the Company or its stockholders, (b) not in good faith or involving a knowing violation of law or (c) resulting in receipt by such person of an improper personal benefit.

The By-Laws of the Company provide that a former, present or future director, officer or employee of the Company or of any of the Company’s subsidiaries or the respective legal representatives of any such director, officer or employee shall be indemnified by the Company to the fullest extent permitted by the laws of the State of New Jersey, from and against any and all liabilities and expenses in connection with any civil, criminal,

II-1

administrative, legislative, or arbitrative action, suit or other proceeding, or any inquiry or investigation that could lead to any such proceeding or any appeal therein in which he or she is or was involved, or is or was threatened to become involved, by reason of being or having been a corporate agent; provided that in connection with any such proceeding by or in the right of the Company, no indemnification shall be provided as to any person adjudged by any court to be liable to the Company except as and to the extent determined by such court; and provided, further, that any indemnification pursuant to the bylaws of the Company in connection with the settlement or other similar nonadjudicative disposition of any threatened or pending proceeding shall only be granted to the extent permitted by law.

The Company enters into indemnification agreements with its directors and officers and enters into insurance agreements on its own behalf. The indemnification agreements provide that the Company agrees to hold harmless and indemnify its directors and officers to the fullest extent authorized or permitted by the Business Corporation Act of the State of New Jersey, or any other applicable law, or by any amendment thereof or other statutory provisions authorizing or permitting such indemnification that is adopted after the date hereof. Without limiting the generality of the foregoing, the Company agrees to hold harmless and indemnify its directors and officers to the fullest extent permitted by applicable law against any and all expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by its directors and officers in connection with the defense of any present or future threatened, pending or completed claim, action, suit or proceeding by reason of the fact that they were, are, shall be or shall have been a director or officer of the Company, or are or were serving, shall serve or shall have served, at the request of the Company, as director or officer of another corporation, partnership, joint venture, trust, employee benefit plan or other enterprise.

II-2

Provided, however, That: paragraphs (i), (ii), and (iii) of this section do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the Commission by the registrant pursuant to section 13 or section 15(d) of the Securities Exchange Act of 1934 that are incorporated by reference in the registration statement, or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the registration statement.

II-3

The undersigned registrant undertakes that in a primary offering of securities of the undersigned registrant pursuant to this registration statement, regardless of the underwriting method used to sell the securities to the purchaser, if the securities are offered or sold to such purchaser by means of any of the following communications, the undersigned registrant will be a seller to the purchaser and will be considered to offer or sell such securities to such purchaser:

(b) The undersigned registrant hereby undertakes that, for purposes of determining any liability under the Securities Act of 1933, each filing of the registrant’s annual report pursuant to Section 13(a) or 15(d) of the Securities Exchange Act of 1934 (and, where applicable, each filing of an employee benefit plan’s annual report pursuant to Section 15(d) of the Securities Exchange Act of 1934) that is incorporated by reference in the registration statement shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

(c) Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to directors, officers and controlling persons of the registrant pursuant to the foregoing provisions, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Act and will be governed by the final adjudication of such issue.

II-4

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Kenilworth, State of New Jersey, on March 29, 2018.

Each person whose name appears below hereby constitutes Michael J. Holston and Geralyn S. Ritter, and each of them severally, such person’s true and lawful attorneys-in-fact and agents, with full power of substitution and resubstitution, to sign for such person and in such person’s name and capacity indicated below, any and all amendments to this registration statement, including post-effective amendments, and to file the same with the Securities and Exchange Commission, hereby ratifying and confirming such person’s signature as it may be signed by said attorneys or any of them to any and all such amendments.

Pursuant to the requirements of the Securities Act, this registration statement has been signed by the following persons in the capacities and on the dates indicated: