Gaps in STEM performance have narrowed over the last 50 years (Leaper et al., 2012; National Science Foundation, 2013). Nonetheless, disparities remain in many STEM fields -- with the largest occurring in the physical sciences, computer science, and engineering (UNESCO, 2012). Although gender gaps in science and math performance have been closing, gaps in STEM self-concept and aspirations continue. We investigated academically advanced 12-13 year-old students' sense of belonging to STEM disciplines and examined the relationship among STEM sense of belonging, STEM classroom/peer climate, amount of STEM exposure, and the strength of self-identification with STEM. Although previous research established that features of the STEM classroom/peer climate can promote as well as detract from underrepresented students' interest and retention in STEM (Riegle-Crumb et al., 2006; Roberts et al., 2018; Stake & Nickens, 2005), less is known about constructs that underlie STEM sense of belonging. Our results indicated no significant difference in STEM sense of belonging between academically advanced females and males. However, there is a statistically significant interaction between gender and self-identification with STEM. Key predictors of STEM sense of belonging identified by our regression model include classroom/peer climate and STEM self-identification.