Research has shown that both gender and family socioeconomic status (SES) are important determinants of adolescents' physical activity (PA). This study expands our current knowledge by exploring the moderating role of family SES on gender differences in adolescents' out-of-school PA. We also examine whether this moderating effect varies across countries with different levels of gender inequality. Based on existing empirical studies and theoretical framework, it is hypothesized that 1) gender differences in out-of-school PA would be smaller among adolescents from higher SES families due to them being more likely to share egalitarian gender norms, and 2) this SES gradient in gender differences in adolescents' out-of-school PA would be more salient in countries with lower levels of gender inequality. A total of 316,657 15-year-old adolescents in 50 countries from the 2015 Programme for International Student Assessment (PISA) survey were analyzed. Our multilevel negative binomial regression models show that the gender gap in out-of-school PA is mitigated among families with higher SES, but this moderating role of family SES is less noticeable in countries with higher levels of national gender inequality. Our findings suggest the importance of egalitarian gender culture at both individual and macro-level in understanding gender disparities in adolescents’ PA.