Teaching mathematics involves helping students develop mathematical skills and empowering students to see themselves as capable of participating in and being knowers and doers of mathematics. Extant research has postulated that mathematics identity is a critical contributor to adolescents' mathematics achievement and subsequent academic success. Guided by motivation and instructional quality theories, this classroom-based study examined a mediating mechanism through which teacher practices associated with mathematics identity through motivational beliefs (i.e., expectancies, task values, and cost value). Participants included 525 sixth-grade students (48.6% male; 64% European American, 34% African American, 2% other race; 58.6% free-or-reduced lunch) in the United States. The findings suggest that competence beliefs and task values, except for cost value, mediated the association between teacher practices and mathematics identity. These mediation pathways also differed by race. The mediating role of mathematics expectancies was stronger for European American adolescents, while the mediating role of mathematics task values was stronger for African American adolescents, though effect sizes were relatively modest. Teachers seeking to develop students' mathematics identity-especially in their minority or stereotyped students-might consider enhancing their sensitivity to students' psychological needs, quality of feedback, and instructional learning supports in their daily interaction with students.