Ecological momentary assessment (EMA) methodology was used to examine the emotional context of nonsuicidal self-injury (NSSI). Forty-seven adolescents and young adults used a novel smartphone app to monitor their emotional experiences, NSSI thoughts, and NSSI behaviors for 2 weeks. Momentary changes in both negative and positive emotions predicted greater intensity of NSSI thoughts at the subsequent assessment, while only increases in negative emotion predicted NSSI behaviors. Immediately following NSSI behaviors participants reported reduced high-arousal negative emotions and increased low-arousal positive emotions, suggesting that NSSI may be an efficient and effective method of regulating emotion. Findings highlight the importance of addressing emotion regulation in NSSI interventions.