<p>Sense of belonging is an important motivational belief that represents individuals’ sense of fit within an environment. However, many women report feeling a lower sense of belonging than men in STEM fields, which may perpetuate gender inequities. This lack of belonging may develop early in a student’s educational trajectory and disproportionately affect girls. This paper offers two key contributions to improve our understanding of gender differences in STEM belonging in K-12 education. First, this paper reports a systematic review examining 50 samples from 46 quantitative studies looking at the development of children and adolescents’ sense of belonging in K-12 STEM classes and fields. These studies offered valuable knowledge about factors supporting belonging and consequences of belonging, with limited or inconclusive evidence about gender differences and development. Second, we also conducted a mega-analysis of <i>N</i> = 6,111 children and adolescents across five studies to provide the most well-powered and rigorous analysis to date on this issue. Intersectional findings on gender gaps in belonging in four STEM fields showed that gender gaps in belonging were strongest for Hispanic/Latine students in middle school, and in middle/high school computer science and engineering. The discussion highlights experimental studies demonstrating causal influences on girls’ STEM belonging, indicating the potential for future interventions to support belonging. It is critically important for future research to extend these findings in ways that promote girls’ belonging and participation in STEM fields, especially the fields in which they are most underrepresented.</p>