Background: Given the high rates of self-harm among adolescents, recent research has focused on a better understanding of the motives for the behavior. Aims: The present study had three aims: to investigate (a) which motives are most frequently endorsed by adolescents who report self-harm; (b) whether motives reported at baseline predict repetition of self-harm over a 6-month period; and (c) whether self-harm motives differ between boys and girls. Method: In all, 987 school pupils aged 14–16 years completed a lifestyle and coping questionnaire at two time points 6 months apart that recorded self-harm and the associated motives. Results: The motive "to get relief from a terrible state of mind" was the most commonly endorsed reason for self-harm (in boys and girls). Interpersonal reasons (e.g., "to frighten someone") were least commonly endorsed. Regression analyses showed that adolescents who endorsed wanting to get relief from a terrible state of mind at baseline were significantly more likely to repeat self-harm at follow-up than those adolescents who did not cite this motive. Conclusion: The results highlight the complex nature of self-harm. They have implications for mental health provision in educational settings, especially in relation to encouraging regulation of emotions and help-seeking.