UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K

(Mark one)

For the fiscal year ended December 31, 2013

or

For the transition period from                      to                     

Commission file number 000-23108 (issuing entity)

Commission file number 033-54804 (depositor and sponsor)

DISCOVER CARD MASTER TRUST I

(Exact name of issuing entity as specified in its charter)

DISCOVER BANK

(Exact name of depositor and sponsor as specified in its charter)

Depositor’s telephone number, including area code: (302) 323-7315

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act: None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  ¨    No  x

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x

Indicate by check mark whether the registrant: (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§ 232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K (§ 229.405) is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    x

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

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Act).    Yes  ¨    No  x

State the aggregate market value of the voting and non-voting common equity held by non-affiliates computed by reference to the price at which the common equity was last sold, or the average bid and asked price of such common equity, as of the last business day of the registrant’s most recently completed second fiscal quarter.

Registrant has no voting or non-voting common equity outstanding held by non-affiliates.

PART I

The following Items have been omitted in accordance with General Instruction J to Form 10-K:

Item 1. Business.

Item 1A. Risk Factors.

Item 2. Properties.

Item 3. Legal Proceedings.

Item 4. Mine Safety Disclosures.

Item 1B. Unresolved Staff Comments.

Not Applicable.

Substitute information provided in accordance with General Instruction J to Form 10-K:

Item 1112(b) of Regulation AB. Significant Obligors of Pool Assets (Financial Information).

Not Applicable.

Item 1114(b)(2) of Regulation AB. Credit Enhancement and Other Support, Except for Certain Derivatives Instruments (Information Regarding Significant Enhancement Providers) (Financial Information).

Not Applicable.

Item 1115(b) of Regulation AB. Certain Derivatives Instruments (Financial Information).

Not Applicable.

Item 1117 of Regulation AB. Legal Proceedings.

In the normal course of business, from time to time, Discover Bank and/or one or more its affiliates have been named as a defendant in various legal actions, including arbitrations, class actions, and other litigation, arising in connection with our activities. Certain of the actual or threatened legal actions include claims for substantial compensatory and/or punitive damages or claims for indeterminate amounts of damages. Discover Bank contests liability and/or the amount of damages as appropriate in each pending matter.

Discover Bank has historically relied on the arbitration clause in its cardmember agreements, which has in some instances limited the costs of, and our exposure to, litigation, but there can be no assurance that Discover Bank will continue to be successful in enforcing the arbitration clause in the future. Legal challenges to the enforceability of these clauses have led most card issuers and may cause Discover Bank to discontinue their use. In addition, bills are periodically introduced in Congress to directly or indirectly prohibit the use of pre-dispute arbitration clauses, and the Dodd-Frank Act authorized the CFPB to conduct a study on pre-dispute arbitration clauses and, based on the study, potentially limit or ban pre-dispute arbitration clauses. Further, Discover Bank and/or one or more of its affiliates are involved in pending legal actions challenging the arbitration clause.

Discover Bank and its affiliates are also involved, from time to time, in other reviews, investigations and proceedings (both formal and informal) by governmental agencies regarding our business, including, among other matters, accounting, tax and operational matters, some of which may result in adverse judgments, settlements, fines, penalties, injunctions, decreases in regulatory ratings or other relief, which could materially impact Discover Bank’s

financial results, increase its cost of operations, or limit its ability to execute its business strategies and engage in certain business activities. Litigation and regulatory actions could also adversely affect Discover Bank’s reputation.

There are two class action cases pending in relation to the Telephone Consumer Protection Act (“TCPA”). The cases were filed in the U.S. District Court for the Northern District of California on November 30, 2011 (Walter Bradley et al. v. Discover Financial Services) and on March 6, 2012 (Andrew Steinfeld v. Discover Financial Services, DFS Services LLC and Discover Bank). The plaintiff in each case alleges that Discover contacted him, and members of the class he seeks to represent, on their cellular telephones without their express consent in violation of the TCPA. The plaintiff in each case seeks statutory damages for alleged negligent and willful violations of the TCPA, attorneys’ fees, costs and injunctive relief. The TCPA provides for statutory damages of $500 for each violation ($1,500 for willful violations). Discover and class counsel entered into a preliminary settlement of both pending class actions. On September 10, 2013, the court granted preliminary approval of the settlement. The settlement remains subject to final approval by the court following completion of notice to the settlement class.

On April 12, 2012, the Attorney General of Hawaii filed a lawsuit against Discover in the Circuit Court of the First Circuit, Hawaii (Hawaii v. Discover Financial Services, Inc., Discover Bank, DFS Services LLC et al.). The lawsuit asserts various claims related to Discover’s marketing and administration of various protection products under Hawaii law. This matter has been resolved with the Attorney General.

On June 28, 2012, the Attorney General of Mississippi filed a lawsuit against Discover in the Chancery Court of the First Judicial District of Hinds County, Mississippi (Mississippi v. Discover Financial Services, Inc., Discover Bank, DFS Services LLC et al.). The lawsuit asserts various claims related to Discover’s marketing and administration of various protection products under Mississippi law. This matter has been resolved with the Attorney General.

On April 17, 2013, the Attorney General of New Mexico filed a lawsuit against Discover in the First Judicial District Court, County of Santa Fe, New Mexico (New Mexico v. Discover Financial Services, Inc., Discover Bank, DFS Services LLC et al.). The lawsuit asserts various claims related to Discover’s marketing and administration of various protection products under New Mexico law. This matter has been resolved with the Attorney General.

PART II

The following Items have been omitted in accordance with General Instruction J to Form 10-K:

Item 5. Market for Registrant’s Common Equity, Related Stockholder Matters and Issuer Purchases of Equity Securities.

Item 6. Selected Financial Data.

Item 7. Management’s Discussion and Analysis of Financial Condition and Results of Operations.

Item 7A. Quantitative and Qualitative Disclosures About Market Risk.

Item 8. Financial Statements and Supplementary Data.

Item 9. Changes in and Disagreements with Accountants on Accounting and Financial Disclosure.

Item 9A. Controls and Procedures.

Item 9B. Other Information.

None.

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PART III

The following Items have been omitted in accordance with General Instruction J to Form 10-K:

Item 10. Directors, Executive Officers and Corporate Governance.

Item 11. Executive Compensation.

Item 12. Security Ownership of Certain Beneficial Owners and Management and Related Stockholder Matters.

Item 13. Certain Relationships and Related Transactions, and Director Independence.

Item 14. Principal Accountant Fees and Services.

Substitute information provided in accordance with General Instruction J to Form 10-K:

Item 1119 of Regulation AB. Affiliations and Certain Relationships and Related Transactions.

Discover Bank, which acts as seller/depositor for Discover Card Master Trust I (the “Master Trust”), depositor and beneficiary for Discover Card Execution Note Trust (the “Note Issuance Trust”) and the sponsor of their securitizations, is a wholly owned subsidiary of Discover Financial Services. Discover Financial Services acquired Discover Bank in January 1985. Discover Bank and the trustee for the Master Trust formed the Master Trust in October 1993. Discover Bank originates and has transferred to the Master Trust, since its formation, the credit card receivables generated under certain designated Discover card accounts. Discover Bank and the owner trustee formed the Note Issuance Trust on July 2, 2007. Discover Bank transferred an undivided interest in the receivables and other assets of the Master Trust, represented by the collateral certificate, to the Note Issuance Trust to support the issuance of notes on July 26, 2007.

Discover Bank acts as master servicer for the Master Trust and is currently the only servicer under the Pooling and Servicing Agreement with respect to the receivables. Discover Bank has outsourced certain servicing functions to the following affiliates: DB Servicing Corporation (“DBSC”) and Discover Products Inc. (“DPI”). DBSC has contracted with BancTec, Inc., a third party service provider, for certain check processing and related services. However, Discover Bank is ultimately responsible for the overall servicing function. DBSC and DPI are held directly by Discover Bank. Discover Bank also acts as calculation agent for the Note Issuance Trust, which is part of the servicing function.

DRFC Funding LLC (“DRFC”) is an indirect subsidiary of Discover Bank. DRFC is a special purpose entity whose purpose is to provide credit enhancement for the benefit of certificateholders of previously issued series of Master Trust certificates. DRFC will not provide credit enhancement for the collateral certificate or the notes unless otherwise specified in the applicable prospectus supplement.

Item 1122 of Regulation AB. Compliance with Applicable Servicing Criteria.

Each of Discover Bank, DPI, DBSC, BancTec, Inc. and U.S. Bank National Association (each, a “Servicing Participant”) has been identified by the registrant as a party participating in the servicing function during the reporting period with respect to the pool assets held by the Master Trust. Each of the Servicing Participants has completed a report on an assessment of compliance with the servicing criteria applicable to such Servicing Participant (each, a “Report on Assessment”) during the Master Trust’s one-month transition period ended December 31, 2012 and year ended December 31, 2013, which Reports on Assessment are attached as exhibits to this Form 10-K. In addition, each of the Servicing Participants has provided an attestation report (each, an “Attestation Report”) by a registered independent public accounting firm regarding its related Report on Assessment. Each Attestation Report is attached as an exhibit to this Form 10-K. None of the Reports on Assessment or the related Attestation Reports has identified any material instances of noncompliance with the servicing criteria applicable to such Servicing Participant described in the related Report on Assessment.

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Item 1123 of Regulation AB. Servicer Compliance Statement.

Each of Discover Bank, DPI, DBSC and BancTec, Inc. has been identified by the registrant as a servicer during the reporting period with respect to the pool assets held by the Master Trust. Each of Discover Bank, DPI, DBSC and BancTec, Inc. has provided a statement of compliance (each, a “Compliance Statement”) with respect to the servicing activities conducted during the Master Trust’s one-month transition period ended December 31, 2012 and year ended December 31, 2013, signed by an authorized officer, and each such Compliance Statement is attached as an exhibit to this Form 10-K.

PART IV

In December 2012, the Board of Directors of Discover Bank approved a change in the fiscal year-end from November 30 to December 31 for the Master Trust and the Note Issuance Trust. This change in the trusts’ fiscal year-end resulted in a one-month transition period in December 2012. Information for this fiscal transition month period is included in the exhibits to this Annual Report on Form 10-K.

Item 15. Exhibits and Financial Statement Schedules.

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SIGNATURES

Pursuant to the requirements of Section 13 of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

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EXHIBIT INDEX

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