ABSTRACT The study aimed to investigate the association between teachers’ instructional support and peer relationships among Norwegian students in grades 5–10 (ages 10–15). The mediating role of student engagement was investigated. Furthermore, we investigated the effect of student behaviour on peer relationships and whether these associations were mediated by instructional support. The quality of instructional support was measured using the Classroom Assessment Scoring System (CLASS) observation protocol and student reports. Peer relationships were measured by self-report in the fall and the spring of an academic year. Student’s mental health problems and reactive aggressiveness were measured in the fall. The analyses were conducted within a Multilevel Structural Equation Models (MSEM) framework. The results indicated that peer relationships were stable from fall to spring at both the classroom and student levels. Observed, but not student-reported, instructional support was associated with peer relationships. Furthermore, when reactive aggressiveness and mental health problems were controlled for, students’ perceived instructional support in the fall was associated with peer relationships in the spring. The results emphasise the importance of teachers’ instructional support for the quality of peer relationships and affective and instructional aspects of teaching do not need to compete.