The Affordable Care Act implemented optional Medicaid expansion starting in 2014, but the association between Medicaid expansion and gynecologic cancer survival is unknown. To evaluate the impact of Medicaid expansion by comparing 2-year survival among gynecologic cancers before and after 2014 in states that did and did not expand Medicaid using a difference-in-difference analysis. We searched the National Cancer Database for women aged 40 to 64 years, diagnosed with a primary gynecologic malignancy (endometrial, ovarian, cervical, vulvar, and vaginal) between 2010 and 2016. We used a quasiexperimental difference-in-difference multivariable Cox regression analysis to compare 2-year survival between states that expanded Medicaid in January 2014 and states that did not expand Medicaid as of 2016. We performed univariable subgroup difference-in-difference Cox regression analyses on the basis of stage, income, race, ethnicity, and geographic location. Adjusted linear difference-in-difference regressions evaluated the proportion of uninsured patients on the basis of expansion status after 2014. We evaluated adjusted Kaplan-Meier curves to examine differences on the basis of study period and expansion status. Our sample included 169,731 women, including 78,669 (46.3%) in expansion states and 91,062 (53.7%) in nonexpansion states. There was improved 2-year survival on adjusted difference-in-difference Cox regressions for women with ovarian cancer in expansion than in nonexpansion states after 2014 (hazard ratio, 0.88; 95% confidence interval, 0.82-0.94; P<.001) with no differences in endometrial, cervical, vaginal, vulvar, or combined gynecologic cancer sites on the basis of expansion status. On univariable subgroup difference-in-difference Cox analyses, women with ovarian cancer with stage III-IV disease (P=.008), non-Hispanic ethnicity (P=.042), those in the South (P=.016), and women with vulvar cancer in the Northeast (P=.022), had improved 2-year survival in expansion than in nonexpansion states after 2014. In contrast, women with cervical cancer in the South (P=.018) had worse 2-year survival in expansion than in nonexpansion states after 2014. All cancer sites had lower proportions of uninsured patients in expansion than in nonexpansion states after 2014. There was a significant association between Medicaid expansion and improved 2-year survival for women with ovarian cancer in states that expanded Medicaid after 2014. Despite improved insurance coverage, racial, ethnic, and regional survival differences exist between expansion and nonexpansion states.