ABSTRACT Despite the recognised importance of student-teacher and student-peer relationships as well as study effort and engagement in predicting academic performance, the interplay between these predictors has received limited attention in higher education research. Building on Study Demands-Resources theories, this longitudinal study explores the favourable impact of student-teacher and student-peer relationships on academic performance (GPA), mediated by study effort and engagement. In total, 613 undergraduate students (68% female; mean age 20 years) from all 13 Dutch public universities completed two online surveys with a 3.5-month interval. Student-teacher and student-peer relationships, study effort, and study engagement were measured at two time points (t1 and t2), while GPA was only measured at t2. Correlation and path analyses revealed that better relationships with both teachers and peers were longitudinally associated with a higher GPA, mediated by more study effort and engagement. This study contributes to the literature by employing longitudinal data, offering stronger causal evidence for processes underlying academic performance. It underscores the importance of fostering positive relationships with both teachers and peers to improve student performance. The findings suggest that educational institutions should implement strategies to strengthen these relationships, thereby enhancing study effort and engagement, ultimately leading to improved academic performance.