In response to recent interest in K-12th students' mathematics identity formation and its implications for achievement, the present study examines the relationship between learners' mathematics identity and student achievement while simultaneously accounting for teacher-enacted instructional practices in mathematics. Drawing from HSLS:2009, a nationally representative dataset of 9th-12th students within the United States, we use multiple linear regression analyses to examine how teachers' mathematical pedagogies and 9th grade students' perceptions of teacher equity are associated, first, with students' mathematics identity, and, subsequently, math achievement after accounting for students' mathematics identity. Ultimately, results from our models reveal a statistically significant relationship between students' mathematics identity and their mathematics achievement, even after accounting for prior achievement and other covariates. Moreover, teachers' reported use of reform-oriented mathematics pedagogy (i.e. developing problem-solving skills, learning how to reason mathematically, and exploring logical structures in mathematics) and students' perceptions of teacher equity are strongly associated with students' mathematics identity. We close with recommendations for future research examining learners' mathematics identity and implications for educational policy and teachers' pedagogical practice.