PURPOSE Medicaid expansion through the Affordable Care Act (ACA) has been shown to improve insurance coverage and early diagnosis of cancer in young adults (YAs); whether these improvements translate to survival benefits remains unknown. We examined the association between Medicaid expansion under the ACA and 2-year overall survival among YAs with cancer. METHODS Using the National Cancer Database, we identified 345,413 YAs (age 18-39 years) diagnosed with cancer in 2010-2017. We applied the difference-in-differences (DD) method to estimate changes in 2-year overall survival after versus before Medicaid expansion in expansion versus nonexpansion states. RESULTS Among all YAs, 2-year overall survival increased more in expansion states (90.39% pre-expansion to 91.85% postexpansion) than in nonexpansion states (88.98% pre-expansion to 90.07% postexpansion), resulting in a net increase of 0.55 percentage points (ppt; 95% CI, 0.13 to 0.96). The expansion-associated survival benefit was concentrated in patients with female breast cancer (DD, 1.20 ppt; 95%CI, 0.27 to 2.12) when stratifying by cancer type and in patients with stage IV disease (DD, 2.56; 95%CI, 0.36 to 4.77) when stratifying by stage. In addition, greater survival benefit associated with Medicaid expansion was observed among racial and ethnic minoritized groups (DD, 1.01 ppt; 95% CI, 0.14 to 1.87) as compared with non-Hispanic White peers (DD, 0.41 ppt; 95% CI, –0.06 to 0.87) and among patients with a Charlson comorbidity score of ≥ 2 (DD, 6.48 ppt; 95% CI, 0.81 to 12.16) than those with a comorbidity score of 0 (DD, 0.44 ppt; 95% CI, 0.005 to 0.87). CONCLUSION Medicaid expansion under the ACA was associated with an improvement in overall survival among YAs with cancer, with survival benefits most pronounced among patients of under-represented race and ethnicity and patients with high-risk diseases.