Background <p>Non-suicidal self-injury (NSSI) is a prevalent maladaptive coping strategy among adolescents, often associated with difficulties in emotional processing.</p> Methods <p>This study investigated the relationship between emotional competence and NSSI in a clinical sample of 93 adolescent females (aged 12–21 years), 47% of whom met DSM-5 diagnostic criteria for NSSI. Dissociation and borderline personality disorder (BPD) traits were examined as potential moderators. Emotional competence and dissociation were assessed using self-report questionnaires, while BPD traits were evaluated dimensionally via a semi-structured clinical interview. Robust linear regression models were employed to examine main and interaction effects.</p> Results <p>Findings revealed that higher emotional competence was generally associated with reduced lifetime NSSI. This protective effect was particularly evident in individuals with high levels of dissociation, especially when self-reported competencies in recognizing one’s own emotions and emotional expressiveness were high. However, a divergent pattern emerged in relation to BPD traits. While emotional competence was protective at low to moderate trait level, it was associated with increased NSSI at high BPD trait levels. This paradoxical effect was especially pronounced when individuals reported high competence in emotion recognition, regulation, and expressiveness.</p> Conclusions <p>These findings suggest that the protective role of emotional competence in reducing NSSI may be limited or even reversed in the context of severe psychopathology. Identifying such moderating effects enhances our understanding of emotional processing in clinical populations and underscores the importance of tailored interventions. Particularly emotion-based therapeutic approaches may need to be adapted for individuals with high BPD symptoms or pronounced dissociative tendencies to improve treatment outcomes.</p>