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; Mage = 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. (PsycInfo Database Record (c) 2024 APA, all rights reserved).