Grounded in Self-Determination Theory, this longitudinal study examined whether learners’ motivation for mathematics predicts the development of arithmetic fluency, above children’s cognitive capacities and the diagnosis of a mathematical learning disability (MLD). Within a one-year timeframe, two waves of data were collected among elementary school children ( $N = 110$ ; $M$ age = 10.46). Using multivariate latent change modelling, results showed that higher baseline autonomous motivation predicted both higher initial arithmetic fluency and stronger subsequent growth, whereas baseline controlled motivation predicted lower initial fluency but not growth. Moreover, within-person increases in autonomous motivation were associated with greater fluency gains, while changes in controlled motivation were unrelated. Importantly, these within-person effects of autonomous motivation remained robust even after controlling for working memory, intelligence, and an MLD diagnosis, highlighting its unique contribution to progress in arithmetic fluency beyond cognitive capacities and mathematical learning difficulties. Fostering autonomous motivation may therefore be a key lever for educators to support learning progress and help all learners, including those who struggle with mathematics, to better reach their full potential.
