Autism spectrum disorder is characterized by patterns of delay and deviance in the development of social, communicative, and cognitive skills that arise in the first years of life. Although many approaches were investigated for the behavioral problems in ASD, investigations of the treatment of inappropriate sexual behaviors (ISB) were still so rare. Mirtazapine has been a promising intervention for the ISB which do not decrease with behavioral interventions recently. In this case, we aimed to present the dramatic benefit of mirtazapine in a 14-year-old autistic boy with inappropriate sexual behaviors.