In recent years, Dweck (2014) and Duckworth (2016) introduced two important concepts in student learning: mindsets and grit. These variables can drastically change the way individuals handle a learning situation, the degree of effort and persistence they display, and eventually, their level of achievement. Recent literature on self-theories recommends using domain-specific rather than domain-general instruments to explain school achievement. Furthermore, child samples are relatively understudied compared to adolescent and youth samples in terms of self-theories and achievement relations. This study focuses on math mindsets, academic grit, and their relation to math achievement at the primary school level. Math mindsets scale, academic grit scale, and a math achievement test were used as data collection instruments. Data were collected from purposively selected 225 4th grade students in Istanbul, Turkey. A cross-sectional causal model has been tested through structural equation modeling (SEM). There was not a direct effect of growth math mindset but a significant negative effect of fixed math mindset on math achievement. The indirect effect of growth math mindset on math achievement mediated by academic grit was positive and significant; the indirect effect of fixed math mindset was non-significant. Finally, the effect of academic grit on math achievement was positive and significant. The SEM model explained 36% of the variance in math achievement and 64% of academic grit. School-based interventions are recommended to promote a growth math mindset and academic grit. Practical and theoretical implications are discussed in detail.