Threats of sexual violence can occur anywhere, including in the family environment. The subject of this study was a fifteen-year-old adolescent girl diagnosed with post-traumatic stress disorder following an act of incest by her stepfather. This study aimed to examine the effectiveness of trauma-focused cognitive behavioral therapy to reduce symptoms of post-traumatic stress disorder in the subject. The study used the single case-single subject A-B-A design. The intervention was trauma-focused cognitive behavioral therapy administered over 12 sessions. Symptoms of post-traumatic stress disorder were measured using the Child Posttraumatic Stress Disorder Symptom Scale (CPSS). Changes in the CPSS scores in the intervention and post-intervention processes showed that the post-traumatic stress disorder score changed from the moderate to the mild category. The decrease in the score was also evidenced by the results of the different pre-intervention, intervention, and post-intervention CPSS scores using Mann-Whitney nonparametric analysis. The study has implications for applying trauma-focused cognitive behavioral therapy to reduce symptoms of post-traumatic stress disorder in adolescent girls who have been victims of incest.