The efficacy of a peer-mediated, theatre-based intervention on social competence in participants with autism spectrum disorder (ASD) was tested. Thirty 8-to-14 year-olds with ASD were randomly assigned to the treatment (n = 17) or a wait-list control (n = 13) group. Immediately after treatment, group effects were seen on social ability, (d = 0.77), communication symptoms (d = -0.86), group play with toys in the company of peers (d = 0.77), immediate memory of faces as measured by neuropsychological (d = 0.75) and ERP methods (d = 0.93), delayed memory for faces (d = 0.98), and theory of mind (d = 0.99). At the 2 month follow-up period, group effects were detected on communication symptoms (d = 0.82). The results of this pilot clinical trial provide initial support for the efficacy of the theatre-based intervention.