The present study utilizes data from the Swedish TIMSS 2023 sample to investigate the relationships between Grade 4 students’ mathematics self-concept, self-efficacy, and achievement in the Number domain, as well as potential gender differences in these relationships. Structural equation modelling with latent mediation was applied. Boys outperformed girls on the Number achievement and reported higher levels of selfconcept and self-efficacy. Results showed that mathematics self-concept, but not selfefficacy, was strongly and similarly associated with Number achievement for both genders. The results further indicate that gender differences in achievement were partially explained by differences in students’ self-concept. Mathematics self-concept is identified as a key factor associated with early Number achievement.