During early and middle adolescence, individuals are at heightened risk of poor body image and subsequent negative mental health outcomes, and the highly visual nature of social media may play a role in this process. It remains unclear, however, if appearance preoccupation on social media-such as appearance-related social media consciousness (ASMC)-influences offline body image, or if preexisting body image concerns influence online appearance preoccupation. The present study investigated between-person differences and potential bidirectional within-person associations in these experiences among eighth grade adolescents in the United States (n = 1,582; ages 11-15 years old; M-age = 13; 47.5% girls, 45.9% boys, 6.5% another gender identity; 37% Latine, 32% White, 18% Black, 7% Asian, 6% another racial/ethnic identity). Participants completed a longitudinal study over three waves within one academic year. Results indicated that within-person increases in ASMC preceded within-person increases in appearance-contingent self-worth and were bidirectionally associated with worse appearance esteem, with no differences in these associations by gender. Among girls only, self-objectification was associated with subsequent within-person increases in ASMC, but not vice versa. Findings indicate that online appearance preoccupation may influence and be reinforced by general body image concerns.