Objectives The purpose of this study was applicating a smartphone app for Acceptance Commitment Therapy(ACT) for social anxiety and examine its efficacy. Methods The ACT smartphone application is a self-help treatment program of 4 sessions that include 6 components of ACT. The present study was conducted on 61 undergraduate students who had subclinical scores in Korean Social Phobia Scale(K-SPS) or Korean Social Interaction Anxiety Scale(K-SIAS). Participants were randomly assigned to treatment group of 30 or control group(waiting-list) of 31. To examine the effect of the program, all participants completed the scales of K-SPS, K-SIAS, Social Anxiety Acceptance and Action Questionnaire (SA-AAQ), Brief Symptom Inventory-18(BSI-18), Psychological Well-being Scale(PWBS). These questionnaires were measured before and after the treatment in both group and 2 weeks after termination in treatment group only. Results The results indicated that the program significantly reduced the level of social anxiety, somatization symptom, depression and anxiety in treatment group. Also, their social anxiety acceptance behavior and psychological well-being were significantly increased. Furthermore, additional improvements of social anxiety level, somatization symptom and social anxiety acceptance behavior were found at 2 weeks follow-up period. Conclusions These results suggest that ACT smartphone application for social anxiety applied in this study can reduce a level of social anxiety efficiently.