Transition to upper secondary education can represent a challenge for students' wellbeing. In this study, we examined how the social support from guardians, peers, and teachers experienced before transition predicts study engagement, sense of relatedness and overall life satisfaction after transition. Survey data (n = 280) were collected from Finnish students in the 9th grade of lower secondary school (T1), and during the first year of their upper secondary school (either in upper secondary or vocational track; T2). The analyses were conducted using structural equation modelling. The results showed that support from teachers in lower secondary school predicted the students' study engagement and overall satisfaction with life after the transition to secondary education. Moreover, peer support before transition was found to predict students' sense of relatedness after transition, while the support from guardians predicted higher life satisfaction. The findings provide new insights on supporting adolescents' well-being throughout school transitions by investing in supportive social resources early in school.