Teachers' mental health has been recognized as relevant to teacher effectiveness, with past work identifying impacts of teachers' mental health on teacher, classroom, and student outcomes. However, much still needs to be understood about the extent to which teachers' mental health is associated with students' learning experiences, including in which learning contexts and among which student groups effects might surface most pointedly. We investigated associations among fourth grade teachers' (N=65) self-reported depressive and anxious symptomatology and their students' (N=805) self-reported behavioral engagement in mathematics, science, and English language arts and whether these associations varied for students based on their enrollment status in a Free and Reduced Meal (FARM) program, a broad indicator of economic disadvantage. Multilevel modeling revealed interaction effects such that, among disadvantaged students, teachers' depressive symptoms were associated with decreased mathematics and science engagement. Results highlight the importance of providing mental health support for teachers, as well highlight teachers' well-being as relevant to issues of equity in elementary STEM education. [This paper was published in "Contemporary Educational Psychology."]