The aim of this study was to investigate whether the child-related competence beliefs of mothers are associated with the development of Finnish adolescents' self-concept of mathematics and literacy ability during their transition from primary to lower secondary school and whether these associations depend on adolescents' level of performance. The results showed that, first, adolescents' self-concept of mathematics and literacy ability decreased over time. The impact of maternal beliefs on the linear trend of the self-concept of mathematics and literacy ability was dependent on the level of students' performance. Mothers' high beliefs buffered against the decrease in adolescents' self-concept of ability in mathematics, but only among high-performing adolescents. In turn, mothers' high beliefs in adolescents' literacy ability were detrimental to the development of low-performing adolescents' self-concept of ability in literacy, whereas mothers' beliefs had no effect on the change in the self-concept of students with average or high literacy performance.