Animal-assisted therapy (AAT) is increasingly used to address impaired social competence in patients with acquired brain injury. However, the efficacy of AAT has not been tested in these patients. We used a randomised, controlled within subject trial to determine the effects of AAT on social competence in patients undergoing stationary neurorehabilitation. Participants received both AAT sessions and paralleled conventional therapy sessions. The patients' social behaviour was systematically coded on the basis of video recordings of therapy sessions. Moreover, mood, treatment motivation and satisfaction was measured during each therapy session. We analysed 222 AAT and 219 control sessions of 19 patients with linear mixed models. Patients showed a significantly higher amount of social behaviour during AAT. Furthermore, patients' positive emotions, verbal and non-verbal communication, mood, treatment motivation and satisfaction were increased in the presence of an animal. Neutral emotions were reduced but no effect was found regarding negative emotions. Our results show that AAT increases aspects of social competence and leads to higher emotional involvement of patients with acquired brain injury, reflected in higher social engagement, motivation and satisfaction during a therapeutic session.