Low retention rates of undergraduate students in Science, Technology, Engineering, and Math (STEM) fields are a persistent problem in the USA. Information and knowledge of STEM topics is valuable in today's modern and global society, and STEM jobs are both lucrative and in high demand. As such, engagement and retention of undergraduate students in STEM is essential to ensure STEM jobs have skilled employees to meet the demands of the twenty-first century. Participants were 1201 first-year undergraduate students from a STEM-focused university. Results from mediation and moderated mediation analyses suggest that incremental beliefs predict a variety of STEM outcomes (STEM interest, sense of belonging in STEM environment, identity compatibility) through STEM efficacy for STEM majors, not non-STEM majors, all of which are associated with STEM engagement and persistence. More specifically, for STEM majors, incremental beliefs predict higher STEM efficacy which then predicts greater STEM interest and sense of belonging in STEM as well as greater perceived identity compatibility between oneself and STEM and between one's gender and STEM. Future directions and implications are discussed.