This systematic review and meta-analysis is the first to evaluate the effects of group-based social skills training (SST) on parent-report social responsiveness in adults with autism spectrum disorder (ASD). A total of 18 studies were included in the narrative review and among them five randomized-controlled trials (n = 145) were included in the meta-analysis. SST had large positive effects on social responsiveness. The narrative review identified that SST could improve patients' outcomes in adults with ASD. These results should be interpreted with caution due to the moderate quality of the existing evidence, which could have inflated effect sizes. The absence of active comparator control groups makes unclear whether improvements at post-treatment are treatment-specific or are attributable to common factors to all psychotherapies.