1533 Background: This study aims to evaluate the impact of the Affordable Care Act Medicaid expansion on melanoma stage at presentation. Methods: 12,667 non-elderly patients (age 40-64) who were newly diagnosed with melanoma and known Medicaid expansion status were identified from The National Cancer Database. We excluded patients with private insurance, Medicare, or other government or unknown insurance. The time period for the cohort spanned from 2010 to 2020 which includes pre-expansion (2010-2013) and post-expansion periods (2014-2020). Year-to-year trend analysis and difference-in-difference (DID) analysis was performed to analyze tumor staging between states with Medicaid expansion (MES) and without Medicaid expansion (non-MES) before and after the expansion. Results: Out of the 12,667 melanoma patients (56% male, 95% white), 41% of patients were uninsured and 59% had Medicaid during 2010 to 2020. The percentage of patients without insurance decreased significantly from 51% in 2010 to 16% in 2020. There were 21% of patients with stage III and 18% with stage IV melanoma. The trend of melanoma stage at presentation over the study period was shown (Table). Notably, the stage IV melanoma at presentation decreased from 21% to 17% from 2010 to 2020 in MES. In contrast, stage IV melanoma at presentation increased from 20% to 23% over the same period of time in non-MES. After adjusting for confounders, DID analysis revealed a statistically significant decrease in stage IV melanoma at presentation (DID -0.04, p < 0.001) as well as a statistically significant decrease in 3 year mortality (DID -0.05, p < 0.001) between MES and non-MES before and after Medicaid expansion. In addition, the use of immunotherapy in stage IV melanoma in MES was significantly higher than non-MES (47% vs 41%, p < 0.001). Conclusions: The findings reveal the positive impact of Medicaid expansion on melanoma stage at presentation and suggest that improved access to healthcare services can facilitate diagnosing early-stage melanoma and decreasing mortality rate. Despite these findings, there are still 16% of patients who still do not have insurance in 2020 which indicates that continued effort is needed to improve the outcomes of these uninsured patients.[Table: see text]