The objectives of this study were to examine the prevalence of non-suicidal self-injury (NSSI) among adolescents with mental health needs and specific factors associated with NSSI among adolescents aged 14 to 18 years who received mental health services in adult mental health facilities in Ontario, Canada. Data on 2,013 adolescents were obtained from the Ontario Mental Health Reporting System using the Resident Assessment Instrument-Mental Health (RAI-MH) and were analyzed using logistic regression. Approximately, 20.2% (407 adolescents) of the sample engaged in NSSI within the last 12 months. Results from multivariate logistic regression indicate that females were 2.19 times more likely to engage in NSSI than males. Intentional misuse of prescription medication emerged as the most important factor associated with NSSI. Other factors found to be associated with NSSI included multiple psychiatric admissions, sexual abuse, use of alcohol, mood disorders (e.g., depression), adjustment disorders, personality disorders and symptoms of depression. The article discusses the implications of the findings, with suggestions for future research.