Emotion regulation has been specifically linked to both non-suicidal self-injury (NSSI) and attempted suicide. It is also known that self-harm is disproportionally higher (30–68.4%) in forensic samples than in the general population, yet knowledge about the association between emotion regulation and self-harm in forensic settings is scarce. The purpose of this study was to describe emotion regulation in a sample of forensic psychiatric patients, to explore dimensions and levels of emotion regulation between forensic psychiatric patients with and without self-harm, and to explore associations between forensic psychiatric patients’ self-reported emotion regulation and self-reported functions of NSSI. A cohort of forensic psychiatric inpatients (N=98) was consecutively recruited during 2016–2020 from a high-security forensic psychiatric clinic in Sweden. Data were collected through the self-report measures Difficulties in Emotion Regulation Scale (DERS) and Inventory of Statements About Self-injury (ISAS). In relation to the first aim, median total and subscales scores for DERS were reported. Results showed a statistically significant difference in emotion regulation between participants with and without self-harm (p=0.004), with a medium effect size (Cohen’s d=0.65) for the DERS total scale. The DERS subscales returned large differences for Impulse (p=0.001, d=0.86), Goals (p=0.014, d=0.58), and Strategies (p=0.012, d=0.54) between participants with and without self-harm. Finally, DERS scores were correlated with both the interpersonal (rs=0.531, p<0.001, n=43) and intrapersonal factors (rs=0.503, p<0.001, n=43) of NSSI as reported on the ISAS. Participants with self-harm (NSSI and/or suicide attempts) demonstrated significantly more difficulties with emotion regulation than those without self-harm. Emotion dysregulation was associated with both interpersonal and intrapersonal functions of NSSI in the participants. We suggest further studies on forensic psychiatric patients’ maladaptive behaviors that focus on substance abuse, self-harm, and aggressive behaviors in relation to the regulation and expression of emotion.