Research suggests that students put more effort into language learning when they feel that it is a voluntary and self-relevant activity or they enjoy the process of mastering that language (i.e., they have a more self-determined orientation). This orientation is fostered when learners feel autonomous, competent, and related to others in their learning environment. We followed 162 university students of French across one semester to examine these causal claims longitudinally. Latent growth curve modeling showed that feelings of autonomy, competence, and relatedness and self-determined motivation increased across the semester while engagement declined. Parallel processes growth curve modeling showed that declines in engagement across the semester were attenuated to the extent that self-determined motivation increased. Auto-regressive cross-lagged analysis showed that, contrary to expectation, more engagement as the semester started predicted greater self-determination mid-semester (instead of vice versa), but these relations became reciprocal from mid-semester on. These findings are consistent with a dynamic model of motivation that emphasizes the reciprocal interplay between motivational constructs over the duration of a language course. The implications of these findings for motivation theory and instructional practices are discussed.