Impaired social functioning is a hallmark feature of autism spectrum disorder (ASD), often requiring treatment throughout the life span. PEERS® (Program for the Education and Enrichment of Relational Skills) is a parent-assisted social skills training for teens with ASD. Although PEERS® has an established evidence base in improving the social skills of adolescents and young adults with ASD in North America, the efficacy of this treatment has yet to be established in cross-cultural validation trials. The objective of this study is to examine the feasibility and treatment efficacy of a Korean version of PEERS® for enhancing social skills through a randomized controlled trial (RCT).The English version of the PEERS® Treatment Manual (Laugeson & Frankel, 2010) was translated into Korean and reviewed by 21 child mental health professionals. Items identified as culturally sensitive were surveyed by 447 middle school students, and material was modified accordingly. Participants included 47 teens between 12 and 18 years of age with a diagnosis of ASD and a verbal intelligence quotient (IQ) ≥ 65. Eligible teens were randomly assigned to a treatment group (TG) or delayed treatment control group (CG). Primary outcome measures included questionnaires and direct observations quantifying social ability and problems directly related to ASD. Secondary outcome measures included scales for depressive symptoms, anxiety, and other behavioral problems. Rating scales for parental depressive symptoms and anxiety were examined to detect changes in parental psychosocial functioning throughout the PEERS® treatment. Independent samples t-tests revealed no significant differences at baseline across the TG and CG conditions with regard to age (14.04 ± 1.64 and 13.54 ± 1.50 years), IQ (99.39 ± 18.09 & 100.67 ± 16.97), parental education, socioeconomic status, or ASD symptoms (p < 0.05), respectively. Results for treatment outcome suggest that the TG showed significant improvement in communication and social interaction domain scores on the Autism Diagnostic Observation Schedule, interpersonal relationship and play/leisure time on the subdomain scores of the Korean version of the Vineland Adaptive Behavior Scale (p's < 0.01), social skills knowledge total scores on the Test of Adolescent Social Skills Knowledge—Revised (p < 0.01), and decreased depressive symptoms on the Child Depression Inventory following treatment (p < 0.05). Analyses of parental outcome reveal a significant decrease in maternal state anxiety in the TG after controlling for potential confounding variables (p < 0.05). Despite cultural and linguistic differences, the PEERS® social skills intervention appears to be efficacious for teens with ASD in Korea with modest cultural adjustment. In an RCT, participants receiving the PEERS® treatment showed significant improvement in social skills knowledge, interpersonal skills, and play/leisure skills, as well as a decrease in depressive symptoms and ASD symptoms. This study represents one of only a few cross-cultural validation trials of an established evidence-based treatment for adolescents with ASD. Autism Res 2014, 7: 145–161. © 2014 International Society for Autism Research, Wiley Periodicals, Inc.