Objectives The purpose of this study is to investigate the effect of art therapy in the improvement of self-ex-pression and interpersonal relationships in adolescents with selective mutism. Methods The research subject is a third-year middle school male student with selective mutism who meets all five criteria of DSM-5 diagnosis. The art therapy of this study was conducted from November 2019 to February 2020, totaling 19 sessions. The research subject received one session per week, each lasting 45 minutes. However, sessions that were conducted in parallel with test and counseling, and the interview counseling of father were ex-tended to 90 minutes each. The parents, who were going through divorce, received individual counseling, family counseling session was conducted once, the father, mother, and a homeroom teacher were provided counseling as needed after the sessions with the subject. The study used HTP for examining changes in self-expression of the research subject with selective mutism, and KSD for exploring changes in relationships. HTP and KSD were evaluated by comparative analysis of pre- and post-test results. The process of change was qualitatively analyzed based on the artwork created during art activities, session transcripts, and counseling journals for each session. Results Firstly, in the post-test of the HTP, the size of the drawings increased by approximately three times, the positioning and composition became more stable, the shading in the human figures disappeared and the depiction of a smiling expression indicated an improvement in self-expression. Secondly, in the post-test of the KSD, the size of the drawings increased by approximately four times, and the positioning, pen pressure, use of an eraser being more stable and the inner aspect of the face being apparent indicated an improvement in interpersonal relationships. Thirdly, throughout the progression across sessions, the subject exhibited emotional stability and genuine self-expression, leading to positive changes in both self-expression and interpersonal relationships. Conclusions considering the combined results, art therapy had demonstrated significant effects on self-ex-pression and interpersonal relationships in adolescents with selective mutism.