Background: This study aimed to empirically derive and validate subtypes of nonsuicidal self-injury (NSSI) based on the forms of self-harm behavior. Methods: Sixty young adults who exhibited repetitive NSSI completed a clinical interview and baseline measures, followed by a two-week ecological momentary assessment (EMA). A latent class analysis was administered to identify NSSI subtypes based on the forms of self-harm behavior. The subtypes were then compared to baseline clinical and EMA variables, including emotions and urges to self-harm. Results: The best-fitting model supported two subtypes: 1) substance abuse and suicide attempt subtype and 2) cutting and scratching subtype. The substance abuse and suicide attempt subtype reported a higher rate of engaging in lifetime suicide plans and suicide attempts and a higher number of lifetime suicide attempts. They also showed significantly higher severity of self-harm behavior, borderline personality traits, anger, posttraumatic symptoms, and difficulties in emotion regulation when compared to the cutting and scratching subtype. However, there was no inter-subtype difference in self-esteem. In the EMA data, anger toward others, feeling of rejection, loneliness, and helplessness were significantly high in the substance abuse and suicide attempt subtype. The variability of anger toward others was also higher in this subtype compared to the cutting and scratching subtype. Finally, in the two-week research period, the substance abuse and suicide attempt group showed a higher rate of reported urges to self-harm with or without suicidal intent. Conclusion: Findings support the validity of the subtypes based on the forms of self-harm behavior, suggesting the clinical utility of addressing heterogeneity within individuals exhibiting self-harm for assessment and treatment.