The Affordable Care Act (ACA) aimed to achieve nearly universal health insurance coverage in the United States through a combination of insurance market reforms, mandates, subsidies, health insurance exchanges, and Medicaid expansions, most of which took effect in 2014. This paper estimates the causal effects of the ACA on health insurance coverage in 2014 using data from the American Community Survey. We utilize difference‐in‐difference‐in‐differences models that exploit cross‐sectional variation in the intensity of treatment arising from state participation in the Medicaid expansion and local area pre‐ACA uninsured rates. This strategy allows us to identify the effects of the ACA in both Medicaid expansion and non‐expansion states. Our preferred specification suggests that, at the average pre‐treatment uninsured rate, the full ACA increased the proportion of residents with insurance by 5.9 percentage points compared to 2.8 percentage points in states that did not expand Medicaid. Private insurance expansions from the ACA were due to increases in both employer‐provided and non‐group coverage. The coverage gains from the full ACA were largest for those without a college degree, non‐whites, young adults, unmarried individuals, and those without children in the home. We find no evidence that the Medicaid expansion crowded out private coverage.