Objectives. To estimate the effect of Medicaid expansion on noncitizens' and citizens' participation in the Supplemental Security Income (SSI) program. The Affordable Care Act (ACA) expanded Medicaid eligibility to cover low-income nonelderly adults without children, thus delinking their Medicaid participation from participation in the SSI program.Methods. Using data from the Social Security Administration for 2009 through 2018 (n = 1020 state-year observations) and the Current Population Survey for 2009 through 2019 (n = 78 776 respondents), we employed a difference-in-differences approach comparing SSI participation rates in US states that adopted Medicaid expansion with participation rates in nonexpansion states before and after ACA implementation.Results. Medicaid expansion reduced the SSI (disability) participation of nonelderly noncitizens by 12% and of nonelderly citizens by 2%. Estimates remained robust with administrative and survey data.Conclusions. Medicaid expansion caused a substantially larger decline in the SSI participation of noncitizens, who face more restrictive SSI eligibility criteria, than of citizens. Our estimates suggest an annual savings of $619 million in the federal SSI cost because of the decline in SSI participation among noncitizens and citizens.