The transfer pathway from community college to university holds promise for advancing equity in STEM because it is followed by disproportionately high numbers of underrepresented students. Among the challenges these students face is cultivating belonging in multiple institutional settings. By combining belonging and validation theories, this qualitative study investigated how underrepresented students' belonging developed in their STEM majors, highlighting differences between students who transferred and those who began as first-time in college (FTIC) students. The findings revealed that for each type of belonging experience a smaller proportion of transfer students than FTIC students experienced validation and a higher proportion experienced invalidation. Department-based transfer student orientation and ongoing programming were uniformly validating to STEM transfer students. The study provides evidence that major belonging is an academic phenomenon that is within the scope of institutional responsibility to improve. Practical implications for administrators and faculty are included as are suggestions for future research.