The Affordable Care Act (ACA), formally known as the Patient Protection and Affordable Care Act and colloquially known as Obamacare, is a landmark U.S. federal statute enacted by the 111th United States Congress and signed into law by President Barack Obama on March 23, 2010. Together with the Health Care and Education Reconciliation Act of 2010 amendment, it represents the U.S. healthcare system's most significant regulatory overhaul and expansion of coverage since the enactment of Medicare and Medicaid in 1965.

The ACA's major provisions came into force in 2014. By 2016, the uninsured share of the population had roughly halved, with estimates ranging from 20 to 24 million additional people covered. The law also enacted a host of delivery system reforms intended to constrain healthcare costs and improve quality. After it went into effect, increases in overall healthcare spending slowed, including premiums for employer-based insurance plans.

The increased coverage was due, roughly equally, to an expansion of Medicaid eligibility and to changes to individual insurance markets. Both received new spending, funded through a combination of new taxes and cuts to Medicare provider rates and Medicare Advantage. Several Congressional Budget Office (CBO) reports said that overall these provisions reduced the budget deficit, that repealing ACA would increase the deficit, and that the law reduced income inequality by taxing primarily the top 1% to fund roughly $600 in benefits on average to families in the bottom 40% of the income distribution.

The act largely retained the existing structure of Medicare, Medicaid, and the employer market, but individual markets were radically overhauled. Insurers were made to accept all applicants without charging based on preexisting conditions or demographic status (except age). To combat the resultant adverse selection, the act mandated that individuals buy insurance (or pay a fine/tax) and that insurers cover a list of "essential health benefits".

Before and after enactment the ACA faced strong political opposition, calls for repeal and legal challenges. In National Federation of Independent Business v. Sebelius, the Supreme Court ruled that states could choose not to participate in the law's Medicaid expansion, but upheld the law as a whole. The federal health insurance exchange, HealthCare.gov, faced major technical problems at the beginning of its rollout in 2013. Polls initially found that a plurality of Americans opposed the act, although its individual provisions were generally more popular. By 2017, the law had majority support. The Tax Cuts and Jobs Act of 2017 set the individual mandate penalty at $0 starting in 2019. This raised questions about whether the ACA was still constitutional. In June 2021, the Supreme Court upheld the ACA for the third time in California v. Texas.