Initiatives promoting diversity, equity, and inclusion in predominantly White contexts, including STEM fields, have primarily relied on approaches to increase the representation of minoritized individuals. However, an increase in the representation of minoritized individuals is only one step of the process, as the present study suggests that explicit beliefs about particular racial groups' abilities also matter. The present article examined whether classroom racial stereotype endorsement about science and math disadvantaged Black American adolescents relative to their White American peers. Across two longitudinal studies with 533 and 1,189 adolescents (N-adolescents = 1722; N-classrooms = 86; 45% Black American, 55% White American; 51% females; M-age = 13-14), classroom pro-White/anti-Black stereotype endorsement in the fall term predicted better science and math achievement scores for White American adolescents and lower science and math achievement scores for Black American adolescents at the end of the academic year. Student- and teacher-reported student engagement in science and math classrooms mediated the longitudinal relations between classroom pro-White/anti-Black stereotype endorsement and students' achievement scores. Results suggest that classrooms may be important conduits for communicating racial stereotypes that create racially hostile STEM learning environments.