Background: The Internet has transformed into a versatile digital platform encompassing social interactions, academic research, knowledge acquisition, and entertainment and resulting in healthcare problems among all age groups, especially adolescents. This study sought to explore the prevalence of problematic internet use (PIU) and its effects on adolescents' mental well-being in the Kingdom of Saudi Arabia. This descriptive, cross-sectional comparative study involved a convenience sample comprising 154 Saudi Arabian adolescents. Data were collected through an online survey, employing the Problematic Internet Use Questionnaire Short Form-6 (PIUQ-SF-6) and the Revised Mental Health Inventory (MHI-5). A substantial proportion of the sample exhibited problematic internet usage patterns, coupled with a moderate level of mental well-being. Except for gender (female) and academic performance (“fair" evaluation grade) in the preceding year, sociodemographic factors exhibited no significant correlation with PIU scores. Mental health scores had a significant negative correlation with PIU; the obsession, neglect, and control subscales; hours spent on the Internet daily, and daily sleeping hours. Conversely, there was a significant positive correlation between PIU and the days per week and hours per day of internet use, as well as with daily sleeping hours. Finally, daily sleeping hours and days per week of internet use significantly predicted adolescents' mental health. To foster adolescent mental well-being, it is imperative to explore this area through continued research efforts. Mitigating excessive online engagement and its associated psychological challenges requires a multilevel approach encompassing individual counseling and family-centered interventions.