Student motivation research seeks to uncover greater understanding of when, how, and why students succeed or fail in school settings. Self-determination theory has been at the forefront of helping educational stakeholders answer questions on student motivation. This study investigates the motivation mediation model proposed by self-determination theory using a longitudinal research design. A total of 1,789 Grade 8 Australian physical education students reported their perceptions of their teacher's motivational style (antecedent), their levels of basic psychological need satisfaction (mediator), their motivation (outcome), and their affect (outcome) across 3 time points. Bifactor exploratory structural equation modeling (bifactor-ESEM) was used to simultaneously test the mediating roles of students' global levels of basic psychological need satisfaction and of the specific satisfaction of their basic psychological needs for autonomy, competence, and relatedness. A longitudinal autoregressive cross-lagged model, allowed us to achieve a systematic disaggregation of the cross-sectional and longitudinal associations between constructs. Findings first supported the superiority of the bifactor-ESEM representation of students' need satisfaction ratings over alternative measurement models, as well as their longitudinal measurement invariance. Second, the longitudinal predictive model revealed that only students' global levels of basic psychological need satisfaction mediated the relations observed between the theoretical antecedents and outcomes in the motivation mediation model. However, meaningful relations between specific factors and outcomes were also identified.