Background: The college years are a developmentally sensitive period for mental disorder onset. Reliable epidemiological data are critical for informing public health responses. This study aimed to estimate prevalence and socio-demographic distributions of common DSM-5 mental disorders among first-year university students from 77 universities across 18 countries. Methods: Data were collected 2017-2023 in the World Mental Health International College Student Initiative with n = 72,288 university students. Online surveys assessed alcohol use, attention-deficit/hyperactivity, bipolar, drug use, generalized anxiety, major depression, panic, and post-traumatic stress disorders with validated screening scales. Socio-demographics included student age, sex at birth, gender modality, sexual orientation, and parent education. Results: The weighted mean response rate was 20.8%. Data were calibrated for differential response rates by sex at birth and age. 65.2% of respondents screened positive for lifetime mental disorders and 57.4% for 12-month mental disorders. Females had higher prevalence of internalizing disorders and males of substance and attentiondeficit/hyperactivity disorders. Older age was associated with lower prevalence of most 12-month but not lifetime mental disorders. Non-heterosexual sexual orientation and identifying as transgender were associated with highest prevalence of most mental disorders. Parent education was for the most part uncorrelated with prevalence. Conclusions: Although prevalence might have been overestimated due to the low response rate and possible screening scale miscalibration, results nonetheless suggest that mental disorders are highly prevalent among first-year university students worldwide and are widely distributed with respect to socio-demographic characteristics. These findings highlight the need to implement effective interventions to better support first-year university student mental health.