The increasing prevalence of problematic internet use has heightened concerns about its adverse impact on internet users’ mental health. Despite reviews investigating the associations between problematic internet use and mental health outcomes, there is a lack of understanding of various aspects of students’ mental health. This study aimed to bridge this gap by providing a more comprehensive overall picture of this issue through a quantitative synthesis based on three-level random-effects meta-analytic models. In total, 223 studies with a cumulative total of 498,167 participants and 512 effect sizes were included in this synthesis. The results showed that problematic internet use was moderately and positively associated with depressive symptoms, anxiety, loneliness, and other mental health outcomes, and negatively related to subjective well-being. Moderator analyses revealed that several study features (i.e., school grade, region, measure of problematic internet use, publication year, and gender) could explain the variations in the findings across individual studies. These research results provide solid evidence for the link between problematic internet use and different mental health outcomes and have implications for future research and interventions on students’ problematic internet use.