Cannabis use is highly prevalent among justice-involved youth and has been linked to trauma exposure. However, the psychological mechanisms underlying this association remain insufficiently understood. The present study examined whether internalizing problems mediated the relationship between cumulative trauma exposure and cannabis use severity in detained youth. Data came from the SCREEN project, which was conducted across seven Juvenile Detention Centers in the Netherlands. The final analytic sample consisted of 361 participants (97.8% male; Mage  = 18.4, SD = 1.9). The study assessed childhood trauma using the Childhood Trauma Questionnaire, internalizing problems using the Brief Problem Monitor–Youth internalizing subscale, and cannabis use severity using the Cannabis Use Disorders Identification Test–Revised. Mediation analyses were conducted using PROCESS Model 4 with bias-corrected bootstrapping (5,000 resamples) while controlling for age and gender. Results indicated a significant indirect effect of cumulative trauma exposure on cannabis use severity through internalizing problems (b = .03, 95% CI [.01, .06]), supporting partial mediation. The findings are consistent with the self-medication hypothesis and suggest that internalizing problems represent a relevant yet often overlooked pathway linking trauma to cannabis use severity in detained youth.