Objectives. To examine whether the expansion of Medicaid under the Affordable Care Act (ACA) decreased the prevalence of severe food insecurity.Methods. With data on adult respondents to the Food Security Supplement to the Current Population Survey in US states for the years 2010 to 2013 and 2015 to 2016, I used a difference-in-difference design to compare trends in very low food security (VLFS) among low-income childless adults in states that did and did not expand Medicaid in 2014 under the ACA.Results. Among low-income, nonelderly childless adults, VLFS rose from 17.4% before ACA to 17.5% after ACA in nonexpansion states, and fell from 17.6% to 15.9% in expansion states. In difference-in-difference analysis, Medicaid expansion was associated with a significant adjusted 2.2-percentage-point decline in rates of VLFS, equivalent to a 12.5% relative reduction.Conclusions. The improvement in food security after the ACA's health insurance expansion suggests that health insurance provision has spillover effects that reduce other dimensions of poverty.Public Health Implications. Providing free or low-cost health insurance coverage may free up household funds, reducing food insecurity and improving this important social determinant of health.