In the context of continued equity gaps in student success within and beyond STEM, this paper explored the extent to which the representation of underrepresented racial minority (URM) and first-generation college students predict grades in postsecondary STEM courses. The analyses examined 87,027 grades received by 11,868 STEM-interested students within 8,468 STEM courses at 20 institutions. Cross-classified multilevel models and student fixed effects analyses of these data both support the same conclusion: the proportion of URM and first-generation students within a class is positively associated with STEM grades among all students, and these relationships are stronger among students who are members of the minoritized group. Thus, promoting the representation of students with minoritized identities in STEM courses may lead to greater equity in college outcomes.