It is argued that the adolescent onset of social anxiety disorder (SAD) may be partly attributable to an increase in avoidance of social situations across this period. The current cohort-sequential study investigated developmental pathways of social avoidance in adolescence and examined the explanatory role of social anxiety and negative cognitive processes. A community sample of youth (9–21 years, N=331) participated in a four-wave study. Trajectory analyses revealed two pathways: an increased avoidance pathway and a low avoidance pathway. The pathways were hardly distinguishable at age 9 and they steadily diverged across adolescence. Logistic regression analyses showed that social anxiety and post-event rumination were significantly related to the increased avoidance pathway; anticipatory processing and self-focused attention were not. The findings suggest that adolescence is a key developmental period for the progression of social avoidance among youth who show relatively high levels of social anxiety and post-event rumination.