<p>Social media activity focused on physical appearance can heighten body dissatisfaction in adolescents. However, the mechanisms behind this association remain insufficiently examined. This three-wave longitudinal study analyzed data from 2500 Czech adolescents (aged 11–16, <i>M</i> = 13.4, <i>SD</i> = 1.7, 50% girls) to examine whether the comparison with social media appearance ideals and the internalization of these ideals mediate the association between appearance activity on social media and body dissatisfaction. The bidirectional relationships and the differences between girls and boys were also explored. While significant between-person correlations were found over time, the within-person results showed that heightened appearance activity did not increase body dissatisfaction in subsequent waves. At the within-person level, social media-ideal internalization and appearance comparison did not mediate this connection. Although the heightened internalization of social media ideals predicted more appearance activity and appearance comparisons at the within-person level, these links were not consistent across waves. No significant differences were observed between adolescent girls and boys. This study indicates that appearance activity on social media do not necessarily reinforce adolescents’ body dissatisfaction six months later, providing insights for both research and policy.</p>