Most adolescents and young adults who experience psychological distress do not seek professional help. This study aims to enhance the understanding of sociodemographic, psychological, and clinical characteristics associated with the underuse of health services by adolescents and young adults with mental disorders. Data from a cross-sectional, epidemiological study with a population-based sample (N = 1180 participants, 14-21 years old) were used. Participants completed a fully standardized, computer-assisted diagnostic interview (DIA-X-5/D-CIDI) administered by trained clinical interviewers to assess lifetime mental disorders according to DSM-5 as well as lifetime health service use for mental health problems, and completed self-report questionnaires to assess various psychological variables (e.g., stigma). Predictors of health service use were examined using univariate and multiple logistic regression analyses, data were weighted for age and sex to improve representativeness Of n = 597 participants with any lifetime mental disorder, 32.4% [95% CI 28.4; 36.7] had ever used any health services because of a mental health, psychosomatic, or substance use problem. Even less had received psychotherapeutic or pharmacological treatment (Cognitive Behavioral Therapy: 12.1% [9.5; 15.2]; other psychotherapy: 10.7% [8.4; 13.7]; medication: 5.4% [3.7; 7.8]). High education was associated with less health service use (low/ middle/ other vs. high education: 53.8% vs. 26.9%; OR = 0.26, p < .001). In the multiple regression model, stigma toward mental disorders was the single psychological variable associated with a reduced likelihood of using health services (OR = 0.69 [0.52; 0.90], p < .01). These findings draw attention to the treatment gap for mental disorders during adolescence and highlight related factors to be addressed in public health contexts.