This study investigated how peer-mediated structured game activities changed and improved peer relations in an inclusive preschool classroom for a child with autism spectrum disorder (ASD). A case study was used in which 17 typically developing (TD) children aged 60-71 months, a 66-month-old boy diagnosed with ASD and the preschool teacher participated. Data were collected before and after peer-mediated structured game activities. The peer tutors engaged in structured activities with the child with ASD for six months. Interviews and observations, picture sociometry, researcher's diary, teacher's diary, and structured play were used to collect the data. The findings revealed significant differences in participants' functional skills before and after the peer-mediated structured game activities. It was observed that the TD children and the boy with ASD improved their social, emotional, and language skills. Children played together for longer. Play time increased, the number of participating children increased. Typically developing children cooperated more and assumed more responsibility. The child with ASD tried to produce meaningful words and participation in play activities was observed. In addition, the peer-mediated structured activities positively supported the teacher's classroom management and classroom climate. In addition, the teacher's level of practical knowledge improved.