Previous research suggests that peer support is an essential component of adolescents' sense of relatedness in peer relationships and correlates with study engagement and life satisfaction. However, what the underlying mechanisms of these relationships are, has remained unclear. This study examined 1) whether the sense of relatedness or study engagement mediate the effect of peer support on students' life satisfaction, and 2) whether sense of relatedness mediates the effects of peer support on students' study engagement. The data were analysed with the structural equation modelling (SEM), using a cross-sectional sample of Finnish upper secondary education students (n = 293) (63% female, 31% male). The results showed that the relationship between peer support and the student life satisfaction beyond the school was fully mediated by students' experiences of study engagement and sense of relatedness. This suggests that peer support for studying by itself is not sufficient for cultivating student life satisfaction, but its effectiveness depends on whether it positively impacts their study engagement and sense of relatedness. To improve the life satisfaction of upper secondary education students, study-related peer support needs to be cultivated so that it triggers students' sense of relatedness and study engagement.