The aim of this study was to assess the reliability and validity of the Amotivation Inventory in Physical Education (AI-PE). In addition, the study sought to identify the relationships between students’ amotivation, physical self-concept, and teacher ratings of National Curriculum attainment levels in PE. Students (N = 510) from a secondary school in South-East England participated in the study by completing a questionnaire during their scheduled PE lesson. Confirmatory factor analysis provided support for the factorial validity of the four-dimensional model of amotivation. Hierarchical multiple regression analyses were conducted on a sub-sample of students with moderate to high levels of amotivation in PE (n = 164). The results showed that both physical self-concept (negatively) and attainment (positively) were predicted by students’ deficient ability beliefs. Overall, the findings support the multidimensionality of amotivation and its association with important outcomes in PE.