Background: With the Affordable Care Act (ACA), the number of uninsured patients in states that expanded Medicaid decreased more among racial/ethnic minorities and lower income adults. Increased access to care could influence colorectal cancer (CRC) incidence through increased screening. However, we lack research on whether Medicaid expansion differentially influenced CRC incidence among vulnerable patient subgroups. This population-based study examines whether Medicaid expansion with the ACA was associated with decreased CRC incidence among racial/ethnic minorities, and adults with lower income. Methods: We queried the Surveillance, Epidemiology, and End Results Program (SEER) database to calculate the age-adjusted incidence rates of CRC among patients under 65 years of age diagnosed between 2010 and 2018. We categorized states into two groups: states that expanded Medicaid on January 1, 2014, and states that did not expand Medicaid over the study period. We determined the change in CRC incidence before Medicaid expansion (2010-2013) and after Medicaid expansion (2014-2018). We used a difference-in-difference approach to determine whether changes in CRC incidence differed by expansion status among all patients and among subgroups stratified by race/ethnicity and other socioeconomic indicators. Results: Among the entire study cohort, from 2010-2013 to 2014-2018, rates of CRC (per 100,000) increased from 26.6 to 28.3, and this increasing rate did not differ by ACA expansion status (p=0.48). We found that the impact of ACA expansion on CRC incidence varied by race/ethnicity. The increase in CRC rates was higher among non-ACA expansion states compared to ACA expansion states for Hispanics (5.4 vs. 1.6 increase per 100,000; p=0.002), and Asian or Pacific Islanders (4.3 vs. 0.4 increase per 100,000; p=0.02), but not with Black (p=0.94), or non-Hispanic white patients (p=0.91). The change in CRC incidence between 2010-2013 and 2014-2018 did not differ by county-level household income, fraction under the federal poverty level, or education level (all p>0.05). Conclusions: This study found that Medicaid expansion through the ACA might differentially benefit Hispanic and Asian patients with respect to decreases in CRC incidence. This study reports on the first 5 years after the ACA, though the true benefits of increased access to care may take longer to manifest. Additional research with longer follow-up is required to fully understand the influence of Medicaid expansion.