Mathematics learning, engagement, and performance are facilitated by quality interactions within the classroom environment. Researchers studying high-quality interactions in mathematics classrooms must consider adopting multiple methods of data collection so as to capture classroom quality from all perspectives. As such, this longitudinal study examined student, teacher, and observer perspectives of interaction quality in mathematics classrooms and their predictive associations with mathematics outcomes. Data were collected during the fall and spring semesters of the 2015-2016 school year from 1501 students in 150 mathematics classes (n = 499 fifth graders, 523 seventh graders, 479 ninth graders; 51% female; 51% European American, 30% African American, and 19% other ethnic background; 52% qualifying for free/reduced price lunch). Observer and aggregated student reports of interaction quality at the classroom level were moderately correlated with one another, and these reports predicted student mathematics engagement and performance. Individual student reports of interaction quality also predicted math engagement and performance; yet, teacher reports of interaction quality did not align with student or observer perspectives. Furthermore, teacher reports did not predict student mathematics outcomes. Implications for research, practice, and policy are discussed.