This study examines the efficacy of the Japanese version of the Program for the Education and Enrichment of Relational Skills (PEERS), which focuses on improving social functioning through making friends and maintaining good relationships for adolescents with autism spectrum disorder (ASD) without intellectual disabilities. Originally developed in the United States, PEERS is one of the few evidence-based social skills training programs for youth with ASD. The present study shows that with linguistic and cultural modifications, PEERS is effective in improving social functioning for adolescents with ASD in Japan. Positive results were found specifically in the areas of socialization, communication, knowledge of social skills, autistic mannerisms, and behavioral and emotional problems. In addition, most treatment gains were maintained at a 3-month follow-up assessment. These findings suggest that the Japanese version of PEERS is beneficial across multiple socio-emotional and behavioral domains for adolescents with ASD.