Self-harm is one of the best predictors of death by suicide, but few studies directly compare adolescents with suicidal versus nonsuicidal self-harm. Seventy adolescents presenting with self-harm (71% young women, ages 12–18 years) who participated in a randomized controlled trial were divided into suicidal and nonsuicidal self-harm categories using the Columbia Classification Algorithm of Suicide Assessment. Adolescents with suicidal self-harm were more likely than those with nonsuicidal self-harm to be young women, 22/23 (96%) versus 34/47 (72%), odds ratio (OR) = 8.33, 95% confidence interval (CI) [1.03, 50.0]; had a later age of onset of self-harm, 15.4 years vs. 13.8 years, mean difference = 1.6, 95% CI [.8, 2.43]; and used self-poisoning more often, 18/23 (78%) versus 11/47 (23%), OR = 3.43, 95% CI [2.00, 5.89]. Only those with nonsuicidal self-harm had an improvement on Children's Global Assessment Scale score following a brief therapeutic intervention, mean difference = 8.20, 95% CI [.97, 15.42]. However, there was no interaction between treatment and suicidality. There are important differences between adolescents presenting with suicidal and nonsuicidal self-harm. Suicidal self-harm in adolescence may be associated with a less favorable response to therapeutic assessment.