Medicare Part D, also called the Medicare prescription drug benefit, is an optional United States federal-government program to help Medicare beneficiaries pay for self-administered prescription drugs. Part D was enacted as part of the Medicare Modernization Act of 2003 and went into effect on January 1, 2006. Under the program, drug benefits are provided by private insurance plans that receive premiums from both enrollees and the government. Part D plans typically pay most of the cost for prescriptions filled by their enrollees. However, plans are later reimbursed for much of this cost through rebates paid by manufacturers and pharmacies.

Part D enrollees cover a portion of their own drug expenses by paying cost-sharing. The amount of cost-sharing an enrollee pays depends on the retail cost of the filled drug, the rules of their plan, and whether they are eligible for additional Federal income-based subsidies. Prior to 2010, enrollees were required to pay 100% of their retail drug costs during the coverage gap phase, commonly referred to as the "doughnut hole.” Subsequent legislation, including the Affordable Care Act, “closed” the doughnut hole from the perspective of beneficiaries, largely through the creation of a manufacturer discount program.

In 2019, about three-quarters of Medicare enrollees obtained drug coverage through Part D. Program expenditures were $102 billion, which accounted for 12% of Medicare spending. Through the Part D program, Medicare finances more than one-third of retail prescription drug spending in the United States.