Social anxiety in adolescence is manifested by anxiety about and avoidance of social interactions. The present study examined whether social anxiety predicts higher levels of both rumination and co-rumination over time. Rumination and co-rumination were studied as possible outcomes because the cognitive content of these processes often involves interpersonal concerns. A three-wave longitudinal study of 575 adolescents (aged 13-16 years old) was conducted over 6 months. Adolescent girls reported higher levels of social anxiety, rumination, and co-rumination than boys. Structural equation modelling analysis showed that social anxiety directly predicted higher levels of rumination and indirectly predicted higher levels of co-rumination over time. A gender difference was noted in that these relationships were more robust for girls than for boys.