Background: Homelessness is associated with an increased incidence of mental illness and risk of self-harm, including suicide. Aims: To assess the prevalence of self-harm (including nonsuicidal self-injury and attempted suicide) among a UK sample of homeless adults and to compare demographic, clinical, and homeless-related variables to determine which are linked to self-harm in this population. Method: A sample of 80 homeless adults were interviewed regarding history of self-harm, mental health history, demographic, and homeless-related information. Results: Sixty-eight percent of the sample reported past acts of self-harm. Those with histories of self-harm started using significantly more substances since becoming homeless and were younger when they first became homeless. They were also significantly more likely to have a past psychiatric admission and thoughts of self-harm in the past year. Conclusion: Self-harm is common among homeless adults and linked to long-term and enduring social and mental health concerns.