<p>Problematic social media use (SMU) is an increasing concern for adolescents’ wellbeing worldwide. The nine-item Social Media Disorder Scale (SMD) is widely used to assess problematic social media engagement in youth, yet its longitudinal psychometric properties remain insufficiently examined. Establishing the scale’s robustness across time is crucial for valid developmental inferences and for informing prevention and intervention efforts. This study evaluated the longitudinal measurement invariance, item-level informativeness and test-level precision of the SMD, as well as its criterion and predictive validity. Self-report data were collected from 306 Greek early adolescents (<i>M</i>_<i>age</i> = 13.30, <i>SD</i> = 0.58 at Wave 1; 42.8% boys) across three waves spanning 12 months. The one-factor structure of the SMD was confirmed and remained invariant across the three waves. Item response theory analyses, estimated separately at each wave, indicated that the scale achieved peak measurement precision at moderate levels of problematic SMU, while several items showed modest fluctuations in informativeness. Higher levels of problematic SMU were consistently associated with elevated depressive symptoms, and, longitudinally, predicted lower daytime alertness, reduced physical activity, and greater energy drink consumption one year later. These findings provide novel evidence for the longitudinal robustness of the SMD, underscoring its utility for developmental research on adolescents’ digital behavior and supporting its application as a screening tool within the Greek context.</p>