Background: This study examines the impact of problematic internet use, or internet addiction, on adolescent mental health, focusing on key psychological variables, assessing the strength of these associations, and identifying potential moderating factors. Methods: A search of the Web of Science databases over the past five years identified 830 articles. Of these, 33 met the inclusion criteria, involving 303,243 participants (average age 14.57; 49.44% female). The selection process was verified by two researchers. Results: Nine psychological variables were analysed: depression, anxiety, stress, suicidal behaviour, psychological well-being, self-esteem, externalising problems, aggressiveness, and impulsiveness. Internet addiction showed positive correlations with aggressiveness (r+ = 0.391), depression (r+ = 0.318), anxiety (r+ = 0.252), and suicidal behaviour (r+ = 0.264). Negative correlations were observed with psychological well-being (r+ = -0.312) and self-esteem (r+ = -0.306). No significant associations were found for externalising problems, impulsiveness, or stress. None of the moderators showed a significant correlation with internet addiction and depression. Conclusions: Although limited by small sample sizes for some variables and the cross-sectional design of most studies, the findings confirm that internet addiction negatively affects adolescents' mental health, being linked to poorer self-perceived health, greater psychological distress, and increased aggression.