Guided by Green-Demers, Leagult, Pelletier, and Pelletier's (2008) assumption that amotivation (absence of motivation) is a multidimensional construct, we designed this study to investigate the influence of inadequate teacher-to-student social support on amotivation of high-school physical education students. Five hundred and sixty-six ninth graders completed questionnaires assessing psychological constructs and intentions for future physical education participation while physical education teachers rated their students' in-class effort. Structural equation modeling analysis revealed that perceived teachers' inadequate supports in autonomy, competence, and relatedness were associated with different subtypes of amotivation. In turn, amotivation impeded in-class effort and intention for future physical education participation. The findings indicate that diminished social support from teachers may act as a significant factor resulting in students' amotivation. The multidimensional nature of amotivation should be identified and instructionally addressed during teaching and learning. (Contains 2 figures and 2 tables.)