<p>Problematic social media use and sleep problems are common concerns among adolescents, yet little is known about how these two constructs are associated across time. This study aimed to examine the longitudinal associations between problematic social media use and sleep problems among adolescents and to explore possible sex differences in these associations. Participants included 1,696 students (Mage = 14.4, SD = 1.5; 47.5% female), with data collected across three waves at six-month intervals. A random-intercept cross-lagged panel model was used to test the within-person associations between the two variables. Multi-group analysis was further conducted to examine potential sex differences. The results indicated a bidirectional association between problematic social media use and sleep problems over time. Specifically, the path from problematic social media use to subsequent sleep problems was significant for both sexes but stronger among females, whereas the reverse path from sleep problems to later problematic social media use was significant only among females. These findings suggest that problematic social media use and sleep disturbances mutually reinforce one another during adolescence, particularly for females, highlighting the importance of sex-sensitive prevention and intervention programs that target both digital behavior and sleep health.</p>