This study examined relationships between language learning motivation and the longitudinal development of second language (L2) pronunciation. Twenty-six English-speaking learners of Spanish recorded a simplified picture description task five times over a yearlong period spanning their second, third, and fourth semesters of Spanish language instruction. Learners also completed a quantitative motivation survey based on the L2 Motivational Self System and an open-ended questionnaire on their language learning beliefs once per semester, yielding three measurements. Eighteen native Spanish listeners rated learners’ clips for comprehensibility and accentedness. Although mixed modeling of the motivation data revealed a slightly negative trajectory for motivational subcomponents, qualitative analyses of individual patterns indicate that learners were beginning to formulate and evaluate language learning goals that were set into a larger framework of personal and professional objectives. Mixed effects models of the pronunciation data demonstrate that both comprehensibility and accentedness improved over time. When the quantitative motivation measures were integrated into the modeling process as time-varying fixed effects, effort was significantly related to accentedness, which suggests that effort may have played an increasingly important role in shaping learners’ pronunciation over time.