Nonsuicidal self-injury (NSSI; i.e., the deliberate destruction of one’s own bodily tissue without suicidal intent and for reasons not socially sanctioned) is prevalent and associated with clinically serious consequences. There is a need for evidence-based, stand-alone treatments for this behavior as it presents across the full range of psychiatric disorders. Developing time-efficient and cost-effective interventions for NSSI has proven difficult given that the core components of treatment remain largely unknown. The aim of this study was to examine the specific effects on NSSI of mindful emotion awareness training and cognitive reappraisal, two transdiagnostic treatment strategies that directly address the functional processes that often maintain self-injury (i.e., relief or escape from aversive thoughts or feelings). Using a counterbalanced, combined series (multiple baseline and phase change) single-case experimental design, the unique and combined impact of these two four-week interventions was evaluated among diagnostically heterogeneous, self-injuring adults (N = 10; mean age = 21.3, range = 18 to 30 years). Hypotheses were that each intervention would produce clinically meaningful reductions in NSSI; adding the alternative intervention would have additive benefit for those who did not respond to the initial intervention alone; and reductions in NSSI would be maintained over a four-week follow-up phase. Results showed that 8 of 10 participants demonstrated clinically meaningful reductions in NSSI by the follow-up phase; six participants responded to one intervention alone, whereas adding the alternative intervention was associated with additive benefit for two participants. Group-based analyses indicated a statistically significant effect of study phase on NSSI (p < .001), with fewer NSSI urges and acts occurring after the interventions were introduced. The interventions were also associated with moderate to large reductions in anxiety (d = 0.89 – 1.09), depression (d = 0.79 – 1.09), and interference caused by symptoms (d = 0.61), and with improvements in skills-based mechanisms: mindful emotion awareness (d = 1.44) and reappraisal (d = 1.30). The results suggest that increasing mindful emotion awareness and cognitive reappraisal may be two key therapeutic strategies for reducing NSSI. Transdiagnostic, emotion-focused interventions delivered in time-limited formats can serve as practical yet powerful treatment approaches, especially for lower-risk self-injuring individuals.