Objectives. Physical activity, including some form of vigorous activity, is a key component of a healthy lifestyle in young people. Self-efficacy and social support have been identified as key determinants of physical activity; however, the mechanism that reflects the interplay of these two factors is not well understood. The aim of the current study was to test social cognitive theory’s notion that self-efficacy relates to intention that translates into behavior and to investigate whether friend support and self-efficacy synergize, interfere, or compensate for one another to predict vigorous physical activity in adolescents—a population at risk of rapid decreases in physical activity. Method. A survey at two points in time was conducted in 226 students aged 12 to 16 years. In a conditional process analysis, friend support and physical activity self-efficacy were specified as interacting predictors of intention. The latter was specified as a mediator between self-efficacy and later vigorous physical activity, controlling for sex and age. Results. Self-efficacy emerged as the dominant predictor of intention, followed by friend support, and an interaction between support and self-efficacy. In adolescents with high self-efficacy, intention was independent of support. In those with low self-efficacy, receiving friend support partly compensated for lack of self-efficacy. The effect of self-efficacy on vigorous physical activity was mediated by intention. Conclusions. Adolescent vigorous physical activity was indirectly predicted by self-efficacy via intention, and this mediation was further moderated by levels of friend support, indicating that friend support can partly buffer lack of self-efficacy.