Objective: This study examined whether the frequency of experiences of ethnic microaggressions and the sensitivity to such experiences were associated with cortisol responses to an acute social stressor (Trier Social Stress Test; TSST) among an ethnically diverse sample of young adults (N = 109, Mage = 18.82 years, SD = 1.40 years, 74% female, 44% Latinx). Method: Self-reported experiences of and sensitivity to microaggressions were assessed using the Everyday Microaggressions Scale. Participants' salivary cortisol was collected before, immediately after, and at three 15-min intervals after the TSST (for a total of 5 salivary samples) to assess their cortisol responses to an acute social stressor. Results: Mixed model analyses revealed that experiencing a higher frequency of microaggressions (p = .005) and being more sensitive to those experiences (p = .001) were associated with a more blunted cortisol response (i.e., lower cortisol reactivity and recovery) to the TSST, relative to experiencing a lower frequency of microaggressions and being less sensitive to them. Furthermore, this blunted cortisol response to the TSST was more prominent among young adults of Latinx and other ethnic backgrounds (i.e., biracial, African American, and Native American) compared to their Asian American and non-Hispanic White peers (p = .034). Conclusion: Findings provide insight into the different ways in which experiences of ethnic microaggressions can be associated with biological markers of stress. (PsycInfo Database Record (c) 2021 APA, all rights reserved).