<p>Although prior research has demonstrated the impact of online sexual objectification on adolescent psychological and behavioral adaptation, the co-development and bidirectional associations between online sexual objectification and non-suicidal self-injury (NSSI) among adolescents remain unclear. The present study investigated the longitudinal co-development of online sexual objectification and NSSI among adolescents. A five-wave longitudinal study including 1,131 Chinese adolescents (<i>M</i>_age = 16.85, <i>SD</i> = 0.62; 47.4% girls) from Henan and Shandong Provinces, with two-year intervals between surveys, was performed. Parallel latent growth modeling indicated that adolescents who initially reported higher levels of online sexual objectification tended to show higher initial levels of NSSI and higher rates of NSSI development over time. Additionally, the rate of online sexual objectification development was positively related to the rate of NSSI development. In contrast, the initial levels of NSSI were negatively correlated with the rate of online sexual objectification development. The random intercept cross-lagged panel model revealed a bidirectional association at the within-individual level, with adolescents who experienced higher levels of online sexual objectification being more likely to exhibit increased NSSI over time and those with higher levels of NSSI tending to experience greater online sexual objectification. These findings highlight the co-development and bidirectional associations between online sexual objectification and NSSI among adolescents and offer empirical evidence for integrated interventions targeting both online sexual objectification and NSSI in the Chinese context.</p>