Medicaid is a government program in the United States that provides health insurance for adults and children with limited income and resources. The program is partially funded and primarily managed by state governments, which also have wide latitude in determining eligibility and benefits, but the federal government sets baseline standards for state Medicaid programs and provides a significant portion of their funding. States are not required to participate in the program, although all have since 1982.

Medicaid was established in 1965, part of the Great Society set of programs during President Lyndon B. Johnson's Administration, and was significantly expanded by the Affordable Care Act (ACA), which was passed in 2010. In most states, any member of a household with income up to 138% of the federal poverty line qualifies for Medicaid coverage under the provisions of the ACA. A 2012 Supreme Court decision established that states may continue to use pre-ACA Medicaid eligibility standards and receive previously established levels of federal Medicaid funding, which led some Republican-controlled states to not expand Medicaid coverage. The 2025 One Big Beautiful Bill Act established requirements that will begin in 2027 for most able-bodied adult Medicaid enrollees to work or volunteer for 80 hours per month in order to maintain coverage.

Medicaid is the largest source of funding for medical and health-related services for people with low income in the United States, providing taxpayer-funded health insurance to 85 million low-income and disabled people as of 2022; in 2019, the program paid for half of all U.S. births. In 2023, the total (federal and state) annual cost of Medicaid was $870 billion, with an average cost per enrollee of $7,600 for 2021. 37% of enrollees were children, but they only accounted for 15% of the spending, ($3,000 per person) while seniors and disabled persons accounted for 21% of enrollees and 52% of spending (more than $18,000 per person). In general, Medicaid recipients must be U.S. citizens or qualified non-citizens, and may include low-income adults, their children, and people with certain disabilities. Medicaid also covers long-term services and supports, including both nursing home care and home- and community-based services, for those with low incomes and minimal assets. Of the 7.7 million Americans who used long-term services and supports in 2020, about 5.6 million were covered by Medicaid.

Along with Medicare, Tricare, ChampVA, and CHIP, Medicaid is one of the several Federal Government-sponsored medical insurance programs in the United States. Medicaid covers healthcare costs for people with low incomes; Medicare is a universal program providing health coverage for the elderly; and the CHIP program covers uninsured children in families with incomes that are too high to be covered by Medicaid. Medicaid offers elder care benefits not normally covered by Medicare, including nursing home care and personal care services. There are also dual health plans for people who have both Medicaid and Medicare.

Research shows that existence of the Medicaid program improves health outcomes, health insurance coverage, access to health care, and recipients' financial security and provides economic benefits to states and health providers. In American politics, the Democratic Party tends to support Medicaid while the Republican Party is divided on reductions in Medicaid spending.

Beginning in the 1980s, many states received waivers from the federal government to create Medicaid managed care programs. Under managed care, Medicaid recipients are enrolled in a private health plan, which receives a fixed monthly premium from the state. The health plan is then responsible for providing for all or most of the recipient's healthcare needs. Today, all but a few states use managed care to provide coverage to a significant proportion of Medicaid enrollees. As of 2014, 26 states have contracts with managed care organizations (MCOs) to deliver long-term care for the elderly and individuals with disabilities. The states pay a monthly capitated rate per member to the MCOs, which in turn provide comprehensive care and accept the risk of managing total costs. Nationwide, roughly 80% of Medicaid enrollees are enrolled in managed care plans. Core eligibility groups of low-income families are most likely to be enrolled in managed care, while the "aged" and "disabled" eligibility groups more often remain in traditional "fee for service" Medicaid.

Because service level costs vary depending on the care and needs of the enrolled, a cost per person average is only a rough measure of actual cost of care. The annual cost of care will vary state to state depending on state approved Medicaid benefits, as well as the state specific care costs. A 2014 Kaiser Family Foundation report estimates the national average per capita annual cost of Medicaid services for children to be $2,577, adults to be $3,278, persons with disabilities to be $16,859, aged persons (65+) to be $13,063, and all Medicaid enrollees to be $5,736.

The Social Security Amendments of 1965 created Medicaid by adding Title XIX to the Social Security Act. Under the program, the federal government provided matching funds to states to enable them to provide Medical Assistance to residents who met certain eligibility requirements. The objective was to help states assist residents whose income and resources were insufficient to pay the costs of traditional commercial health insurance plans.