© 2020 Human Kinetics, Inc. Background: Regular physical activity and doing well in school are important for growing adolescents. In this study, the associations between physical activity and perceived school performance (PSP) are examined together. Methods: Young adolescents from 42 countries (n = 193,949) in Europe and Canada were examined for associations between self-reported moderate to vigorous physical activity (MVPA) and PSP. Multinominal analyses were conducted with 0 to 2 days of MVPA and below average PSP as reference categories. Adjusted odds ratios and 95% confidence intervals were reported for pooled data and individual countries after controlling for family affluence scale. Results: Girls had better PSP than boys, yet more boys participated in daily MVPA than girls. The associations between PSP and MVPA were inverted U shaped. The strongest association for very good PSP was among young adolescents who reported 5 to 6 days MVPA (odds ratios = 2.3; 95% confidence interval, 2.1¿2.4) after controlling for family affluence scale. Conclusions: Young adolescents with average or better PSP took part in at least 3 days of MVPA in a week, suggesting that participating in some MVPA was positively associated with PSP. More days of MVPA in a week, especially for young adolescents with below average PSP, would be beneficial for health and school performance.