Background: Previous research has called for improving psychological interventions and developing new treatments for prisoners. Animal-assisted prison-based programmes have increasingly been used as an approach, but there is a lack of studies investigating the effectiveness of such programmes. Objective: To investigate the effects of a dog-assisted social- and emotional-competence training on the socioemotional competences of prisoners compared to treatment as usual. Methods: In a controlled trial, we investigated 62 prisoners that participated in either a 6-month dog-assisted psychotherapeutic programme or the standard treatment. We assessed social and emotional competences before and after the training and at a 4-month follow-up. Data were analysed with linear models. Results: The prisoners’ self-assessed social and emotional competences did not differ. The psychotherapists rated the prisoners’ emotional competences in the intervention group higher at the follow-up but not after the training. The psychotherapists did not rate the prisoners’ social competences in the intervention group differently but did find them to have higher self-regulation at follow-up and lower aggressiveness after the training than the control group. Conclusions: This study indicates that dog-assisted programmes with a therapeutic aim might be beneficial for prisoners. However, the inconsistent results indicate that more research is needed to determine the potential and limits of animal-assisted programmes in forensic settings.