Background <p>Sleep disturbances and problematic internet use are important public health issues that may be more pronounced in individuals with major depressive disorder.</p> Aim <p>This study aims to investigate sleep disturbance and problematic internet use levels in adolescents with and without major depressive disorder.</p> Methods <p>This study was conducted with 177 adolescents who were admitted to a university training and research hospital between July 1, 2024 and January 30, 2025. The data collection was conducted using the “Adolescent Information Form,” the “Problematic Internet Use Scale-Adolescent Form,” the “DSM-5 Level 2 Sleep Disturbance Scale,” and the “Revised Child Anxiety and Depression Scale-Child Form.” Data were analyzed using Spearman correlation analysis and Multiple regression analysis.</p> Results <p>The case group exhibited significantly higher mean scores than the control group on problematic internet use and sleep disturbances (<i>p</i> &lt; 0.001). The predictors of problematic internet use were sleep disturbance (β = 0.237) and weekly internet usage time (β = 0.148, <i>p</i> &lt; 0.05). The predictors of sleep disturbance in adolescents were child anxiety and depression (β = 5.556), total anxiety (β = 5.044), needing something specific to fall asleep (β = 3.182), and problematic internet use (β = 2.597, <i>p</i> &lt; 0.05).</p> Conclusions <p>Findings indicate a strong and bidirectional relationship between problematic internet use, sleep disturbances, and depressive symptoms in adolescents. Health professionals should routinely screen adolescents for problematic internet use, sleep disturbances, anxiety, and depression, and provide education on healthy digital habits and sleep hygiene.</p>