This study aimed to examine the influence of teacher support on student engagement in mathematics. A cross-sectional survey was employed to recruit 632 students from grades 7 to 9 in junior middle schools across China. Participants completed various scales, including a teacher support scale, an academic self-efficacy scale, and a student engagement scale. Data analysis was conducted using SPSS 20.0 and Amos 24.0. Results showed that, first, emotional support and competence support had a significant positive effect on student engagement in mathematics. The contribution from competence support was the strongest. The contribution from autonomy support was the weakest; second, academic self-efficacy mediated the relationship between autonomy support, emotional support, and competence support and student engagement in mathematics. These findings supplement and expand the understanding of the factors that affect student engagement and provide theoretical support and useful guidance for formulating effective education policies to improve student engagement in mathematics.