It has repeatedly been found that temperamental inhibition and low academic achievement are associated with each other: children with cautious and wary or shy behaviour are at risk for low academic achievement. Several suggestions about the mechanism behind this association have been made, these highlighting for example, the fewer learning opportunities of cautious and wary children and more negative interaction between teachers and inhibited children. However, the empirical studies about these mechanisms are rare and, thus, they have remained unclear. This study examined whether children's maths-related self-concept of ability acts as a mediator between their temperamental inhibition and maths performance. 156 children (M[subscript age] 7.25 years) were followed during the first grade of primary school. Children's temperamental inhibition was assessed in the beginning of Grade 1. Their maths performance was tested twice, in the beginning and at the end of Grade 1, and their self-concept of ability was measured at the end of Grade 1. The research question was analysed using structural equation modelling. The results showed that children's self-concept of ability did mediate the association between temperamental inhibition and maths performance at Grade 1: that more inhibited children feel they are less capable and competent in maths than less inhibited children, and this contributes to their poorer maths performance. The findings highlight that it is important for teachers and other practitioners to be aware of this effect of temperamental inhibition on self-concept and put effort on promoting positive views of children's competencies and abilities.