Background: Little is known about whether Medicaid expansion under the Affordable Care Act (ACA) affected insurance coverage among adult survivors of childhood cancer, a population at high-risk for poor health outcome. We addressed this gap by evaluating the association between ACA Medicaid expansion and Medicaid enrollment among participants in the Childhood Cancer Survivor Study (CCSS). Methods: The CCSS cohort of 5-year survivors of childhood cancer was linked to administrative Medicaid insurance data in 2010-2016. We identified 13,895 adult survivors (aged 18-64 years) diagnosed with cancer under age 21 and between 1970 and 1999. Outcomes included (1) percentage with any Medicaid enrollment by year; and (2) Medicaid-covered days (number of days when a survivor was enrolled in Medicaid) during each year. Multivariable difference-in-differences (DD) models were used to examine outcome changes pre vs. post ACA Medicaid expansion, in expansion- vs. non-expansion states, adjusting for age, sex, race/ethnicity, income, education, and chronic conditions. Multivariable models were conducted overall and then stratified by cancer type, race/ethnicity, income, and education. Results: Medicaid enrollment rates increased more in expansion states (17.6% pre-expansion to 24.1% post-expansion) than non-expansion states (16.4% to 16.9%), leading to a net increase in enrollment of 6.6 percentage points (ppt; 95% CI = 5.5-7.7) in the multivariable DD model. Multivariable DD model showed a net increase of 18.4 days (95% CI = 13.8-23.1) in Medicaid-covered days in expansion states relative to non-expansion states. The expansion-associated increase in Medicaid enrollment rates was greatest among survivors of leukemia (multivariable DD estimate: 8.9 ppt, 95% CI = 6.9-11.0) and non-Hodgkin lymphoma (8.0 ppt, 95% CI = 5.0-10.9). Greater increases in Medicaid enrollment were seen in non-Hispanic Black (13.5 ppt, 95% CI = 8.0-19.1) and Hispanic survivors (15.8 ppt, 95% CI = 10.6-21.0) than non-Hispanic White peers (5.1 ppt; 95% CI = 4.0-6.2); in survivors with < $20K household income (11.7 ppt, 95% CI = 8.9-14.4) compared to those with ≥$60K household income (2.9 ppt, 95% CI = 1.6-4.2); and in survivors with high school or lower education (9.3 ppt, 95% CI = 6.9-11.8) compared to those with a college or higher degree (3.9 ppt, 95% CI = 2.6-5.3). Similar patterns were observed across survivor subgroups when examining Medicaid-covered days. Conclusions: We provide the first evidence on increased Medicaid enrollment and longer coverage duration among adult survivors of childhood cancer following Medicaid expansion, with greater increases seen among survivors of underrepresented racial/ethnic populations, those with low socioeconomic status, and high medical need survivors.