The current research investigates people’s perceptions of others’ lay theories (or mindsets), an understudied construct that we call meta-lay theories. Six studies examine whether underrepresented students’ meta-lay theories influence their sense of belonging to science, technology, engineering, and math (STEM). The studies tested whether underrepresented students who perceive their faculty as believing most students have high scientific aptitude (a universal metatheory) would report a stronger sense of belonging to STEM than those who think their faculty believe that not everyone has high scientific aptitude (a nonuniversal metatheory). Women PhD candidates in STEM fields who held universal rather than nonuniversal metatheories felt greater sense of belonging to their field, both when metatheories were measured (Study 1) and manipulated (Study 2). Undergraduates who held more universal metatheories reported a higher sense of belonging to STEM (Studies 3 and 4) and earned higher final course grades (Study 3). Experimental manipulations depicting a professor communicating the universal lay theory eliminated the difference between African American and European American students’ attraction to a STEM course (Study 5) and between women and men’s sense of belonging to STEM (Study 6). Mini meta-analyses indicated that the universal metatheory increases underrepresented students’ sense of belonging to STEM, reduces the extent of social identity threat they experience, and reduces their perception of faculty as endorsing stereotypes. Across different underrepresented groups, types of institutions, areas of STEM, and points in the STEM pipeline, students’ metaperceptions of faculty’s lay theories about scientific aptitude influence their sense of belonging to STEM.