Objective: To explore whether basketball player's self-determined motivation interacts with environmental contexts and coach training to influence percentage time in moderate-to-vigorous physical activity (%MVPA). Methods: A secondary analysis of data from 76 girls (mean ± SD, 10.5 ± 1.0 years) was conducted. Players were classified as high self-determined motivation (HSDM) or low self-determined motivation (LSDM) and randomised to trained (intervention) or untrained (control) coaches. Training included 2 workshops on strategies for activity-promoting practices. Girls were exposed to environmental contexts (practices and games) led by a trained/untrained coach (depending on arm) and one without coaches (free time) daily. Girls wore accelerometers each day. Using mixed random-effects models, the influence of motivation, context and training on %MVPA was analysed. Results: Trained coaches' practices were associated with the greatest %MVPA with no difference between HSDM and LSDM players (38.28 ± 1.77%; 37.64 ± 1.80%; p = 0.66). HSDM players had significantly greater %MVPA versus LSDM players during untrained coaches' practices (23.58 ± 1.77%; 20.51 ± 1.78%; p = 0.03). During games with trained coaches, HSDM players had greater %MVPA compared to LSDM players (23.79 ± 1.76%; 18.56 ± 1.74%; p < 0.001). No between-group difference in %MVPA during free time was found (12.85 ± 0.82%; 13.39 ± 0.84%; p = 0.64). Conclusion: The impact of individual differences in self-determined motivation on %MVPA during practices was attenuated when coaches were trained to implement activity-promoting practices.