Abstract Despite the well acknowledged importance of motivation in second language (L2) learning, longitudinal research regarding the effects of motivation on growth in L2 proficiency remains limited. Furthermore, limited investigation has been done on how motivation and L2 proficiency interactively develop and affect each other. Thus, this study examined the impact of motivation on growth in kanji proficiency and the dual developmental trajectories of motivation and kanji proficiency using the self-determination theory. Learners of Japanese with Chinese as their First language (L1) responded to a questionnaire and took kanji tests three times in one academic semester (n = 192). The results of the univariate latent growth curve (LGC) modeling identified intrinsic motivation and introjected regulation as positive and negative predictors, respectively, of kanji proficiency growth, suggesting the importance of enjoyment and the detrimental nature of introjected regulation in kanji learning by L1 Chinese learners. The results of multivariate LGC modeling demonstrated a lack of reciprocal developmental association between kanji proficiency and intrinsic motivation. Moreover, perceived competence rather than actual growth in kanji proficiency influenced the enhancement of intrinsic motivation, indicating the importance of subjective interpretations of L2 development in promoting motivation.