Disparities between racially minoritized (Black/Hispanic/Native American) and nonminoritized (White/Asian) students in educational outcomes—both in terms of opportunity and achievement—continue to characterize the American educational system. Social belonging interventions, which shift student construal of social adversity experienced in school, have been shown to be effective with college students (Walton & Cohen, 2011) but have not been tested with high school students. We developed and tested such an intervention in a randomized controlled trial with students transitioning from middle school into high school. Participants (N = 162; 18% minoritized, 79% nonminoritized, 57% women) were randomly assigned to receive intervention or control materials during an optional summer orientation program. By the end of their first year, students in the treatment condition had 46% fewer unexcused absences (95% CI [−66%, −15%]), 36% fewer tardiness records (95% CI [−63%, 12%]), and gained more friends (d = 0.59) than did participants in the control condition. The intervention was particularly effective for racially minoritized participants: Those in the treatment condition failed fewer classes (d = −0.86), earned higher grades (d = 0.90), and received 82% fewer disciplinary citations (95% CI [−97%, 26%]) by the end of the academic year than did their minoritized peers in the control condition. These gains represent reductions in racial gaps ranging from 86% to 100%. Our findings indicate that social belonging interventions might partially redress racial equity gaps in secondary education. Discussion centers on the potential psychological and behavioral mechanisms underlying the intervention.