Previous studies have shown that mastery-approach and performance-approach goals can be positively associated with adaptive educational outcomes. Few studies, however, have examined links with behavioural engagement. The aim of the present study was to examine whether behavioural engagement mediated relations between mastery-approach and performance-approach goals and subsequent achievement in mathematics. Data were collected from 1,057 students aged 9 to 11 years in a longitudinal design over the course of a single school year. Results showed that a mastery-approach, but not a performance-approach, goal predicted behavioural engagement. Behavioural engagement, in turn, predicted mathematics achievement. Furthermore, behavioural engagement mediated relations between mastery-approach and subsequent mathematics achievement. This study contributes to the evidence base for the adaptive role of mastery-approach which can be encouraged by students setting personal best goals, and teachers ensuring that feedback is task-focused, and that the classroom climate is mastery-focused.