Adolescent defending of peer victimization in the school and cyber context has received increased attention in developmental science and is an important component of antibullying interventions. However, the overall prevalence, and individual characteristics that correlate with defending in adolescence, have not been systematically and statistically reviewed. Framed in Bronfenbrenner's social-ecological theory, this meta-analytic review included 172 reports out of 155 studies of defending including 150,978 children and adolescent participants from 4 continents (i.e., North America, Europe, Asia, and Australia) to analyze two aspects: (1) the average proportion of defenders in the population and (2) associations between defending and individual and peer-relational correlates of defending in school and cyber contexts. Using mixed-effects modeling, our results confirmed prior findings of gender differences (favoring girls) and age differences (favoring younger children) in defending. We found positive correlations between defending and affective empathy, cognitive empathy, experiences of peer victimization, self-efficacy, popularity, and acceptance, and a negative correlation between defending and moral disengagement. We also found substantial heterogeneity in these effect sizes. The reporter of defending consistently moderated all mean effect sizes. Implications for prevention efforts and future research are discussed. Public Significance Statement This meta-analytic review is the largest-scale comprehensive review of correlates of children and adolescents' defending others from peer victimization. Small, significant associations between defending and correlates investigated by most researchers were observed. These results suggest that researchers are largely on track in their understanding of characteristics of defenders and that defending researchers should consider these methodological differences and participant characteristics when interpreting their findings.