Medicare Part D, part of Medicare (United States), is a prescription drug plan for the elderly and disabled in the USA. It was enacted as part of the Medicare Prescription Drug, Improvement, and Modernization Act of 2003 (MMA). The plan started on January 1, 2006.

The drug benefit will not be part of the traditional Medicare program, but rather will be offered through private insurance plans that will be reimbursed by the Centers for Medicare & Medicaid Services (CMS). Medicare beneficiaries will have to affirmatively choose and enroll in a Part D plan. Beneficiaries can obtain the Medicare Drug plan through two types of private plans, beneficiaries can join a prescription Drug Plan (PDP) for drug coverage only or they can join a Medicare Advantage (MA) plan that covers drugs and all other Medicare benefits. There will be 34 PDP regions and 26 MA regions in the U.S. The drug plans will control drug costs through a system of tiered formularies in which lower cost drugs are assigned to lower tiers and thus are easier to prescribe.

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