Self-harm is prevalent in incarcerated adults, yet comparatively few studies of self-harm in detained youth (and even fewer in low- and middle-income countries) have been published. We examined the prevalence and correlates of self-harm in a sample of 181 young people (mean age 15.0 years, SD = 2.3) detained in the youth justice system in Sri Lanka. Structured face-to-face questionnaires assessed demographic characteristics, family and social background, substance use, self-harm history (including frequency, method, and intention), bullying victimization, physical and sexual abuse (victimization and perpetration), and exposure to self-harm/suicide by others. Seventy-seven participants (43%) reported a lifetime history of self-harm, 19 of whom (25%) who reported doing so with suicidal intent. Fifty participants (65% of those with a history of self-harm) reported engaging in self-harm impulsively, with no prior planning. A history of self-harm was associated with being female, prior sexual abuse victimization, prior exposure to self-harm by friends, and a lifetime history of self-harm ideation. High rates of substance use, bullying victimization, parental incarceration, and exposure to suicide were reported across the sample. Young people detained in the youth justice system in Sri Lanka are a vulnerable group with high rates of self-harm, substance use, and psychosocial risk factors. Strategies for identifying and preventing self-harm, and targeted psychological interventions designed specifically to address impulsivity, may contribute to more positive outcomes in this marginalised population.