Research adopting self-determination theory (SDT) supports a mediation model whereby coach motivational styles (autonomy support and interpersonal control) predict athletes’ engagement and disaffection in youth sport via the satisfaction and frustration of psychological needs (autonomy, competence, and relatedness). Our study extends this research by examining SDT’s mediation model longitudinally with three-waves of data. Two-hundred and fifty-two youth sports participants (Mage = 12.98; SD = 1.84; range = 11-17; female n = 67) completed measures of study variables at the start, middle, and end of a competitive soccer season. Cross-lagged path analyses revealed that associations between the two coach motivational styles and athletes’ engagement were mediated by psychological need satisfaction. Furthermore, a positive reciprocal association between psychological need satisfaction and engagement emerged over time. This study therefore supports the temporal assumptions underpinning SDT’s mediation model but, importantly, evidences a mutually reinforcing interplay between athletes’ psychological needs and their engaged behavior.