Supplemental Instruction (SI) is a form of structured peer guidance attached to a specific course, provided by an experienced and trained student to a group of students. Previous studies show a positive effect of SI on learning outcomes, some found effects on well-being, and sense of belonging. However, literature on SI lacks randomized controlled trials and does not fully address the risk of self-selection bias. The current study tested whether SI has an effect on grades, mental well-being, and sense of belonging with a pre-registered randomized field experiment and a sample of 493 Dutch first-year students. Students who were offered SI obtained significantly higher grades (d = 0.26) but did not score significantly different on mental well-being or belonging.