Mounting evidence suggests teacher-child race/ethnicity matching and classroom diversity benefit Black and Latinx children's academic and socioemotional development. However, less is known about whether the effects of teacher-child matching differ across levels of classroom diversity. This study examined effects of matching on teacher-reported child outcomes in a racially/ethnically diverse sample of teachers and children, and classroom diversity moderation using multilevel models. Data were drawn from a professional learning study involving 224 teachers (M[subscript age] = 41.5) and 5,200 children (M[subscript age] = 7.7) in 36 New York City elementary schools. Teacher-child race/ethnicity matching was associated with higher child engagement in learning, motivation, social skills, and fewer absences. Classroom diversity moderated matching such that teacher-child mismatch was related to lower engagement, motivation, social skills, math and reading scores in low-diversity classrooms, but not in high-diversity classrooms. Implications for practice and policy are discussed.