The main goal of this research was to assess whether satisfying basic psychological needs (BPN) in a Portuguese school-based mentoring (SBM) program improved the mentored students’ well-being. One uncommon feature of this program was that the mentors were also teachers of the mentees. A two-wave experimental study design was implemented to compare the mentored (n�=�157) and non-mentored students (n�=�160). Surveys were completed twice, with a 6-month interval between time points 1 and 2. The results indicate that SBM that was associated with increased support for BPN was the most effective condition for promoting academic well-being, particularly in terms of improving the mentored students’ perceptions of their school environment. However, no significant differences between the mentored and non-mentored students were detected regarding personal well-being (e.g., psychological well-being) and social well-being (e.g., peer support). These findings suggest that the balanced support of BPN was important in this particular SBM program. This support and enduring mentoring relationships can facilitate the general improvement of the mentored students’ well-being.