In German forensic psychiatry, sports therapy denotes an accompanying therapeutic treatment, although evidence about specific psychometric and mental health outcomes is lacking. This article provides an important contribution to sports therapy interventions. It focuses on one of the most important sports therapy goals for forensic patients: social competence. For this purpose, the intervention ‘Group Dynamics in Volleyball Program’ (GDiVP) was developed, which included practice of three components of social competence: perspective adoption, communication skills, and social responsibility. This study included a pre-post control group design. According to the results, GDiVP improved all three of the components of social competence mentioned above. Further studies must determine how sustainable the positive changes are and to what extent the intervention can be transferred to other sports.