While significant mental illness stigma disparities across race/ethnicity and gender exist, little is known about the efficacy of antistigma interventions in reducing these intersectional disparities. We examine the 2-year effects of school-based antistigma interventions on race/ethnic and gender intersectional stigma disparities among adolescents. An ethnically and socioeconomically diverse sixth-grade sample (N = 302) self-completed surveys assessing stigma before randomly receiving an antistigma curriculum and/or contact intervention versus no intervention. Surveys were also self-completed 2-year postintervention. Stigma measures assessed general mental illness knowledge/attitudes, awareness/action, and social distance. Stigma toward peers with specific mental illnesses was examined using vignettes—two adolescent characters were described as having bipolar (Julia) and social anxiety (David) disorder. Race/ethnicity and gender were cross-classified into six intersectional groups (Latina/o, non-Latina/o Black, and non-Latina/o White girls and boys). Linear regressions adjusting for poverty and mental illness familiarity examined antistigma intervention effects across intersectional groups in sixth and eighth grade. The school-based antistigma intervention reduced intersectional stigma disparities over the 2-year study period. While non-Latino Black boys and Latino boys/girls reported greater disparities in stigma at baseline compared to non-Latina White girls, these disparities (14 total) were predominantly eliminated in the 2-year follow-up following receipt of the curriculum and contact components to just one remaining disparity postintervention among non-Latino Black boys. By identifying differences in how school-based antistigma interventions reduce mental illness stigma for unique race/ethnic and gender intersectional groups, we can better understand how to shape future antistigma interventions for diverse intersectional populations.