<p>Sexual objectification has been linked to self-objectification and increased mental health risks. With the growing prevalence of social media, it remains unclear who engages in online sexual objectification and what psychological mechanisms drive these behaviors, particularly among adolescents. This study examined the associations between ambivalent sexism, thin-ideal internalization, and online sexual objectification using a 13-month longitudinal design. A sample of 577 Chinese adolescents (aged 11–14 years; 270 girls, 307 boys) completed self-report measures at baseline (T1) and again at a 13-month follow-up (T2). Cross-lagged panel analyses revealed that T1 benevolent sexism predicted increased T2 thin-ideal internalization and T2 online sexual objectification. However, T1 online sexual objectification was not significantly associated with T2 thin-ideal internalization or T2 benevolent sexism. Moreover, thin-ideal internalization mediated the relationship between benevolent sexism and online sexual objectification. These findings suggest that benevolent sexism may be a key risk factor for adolescent objectification on social media. Interventions targeting benevolent sexism may help reduce online sexual objectification and its adverse effects on adolescent girls.</p>