Background and Purpose: Autism spectrum disorder (ASD) affects communication skills and appropriate response to the outside environment. One of the newest ways to improve the communication skills of children with autism spectrum disorder is hydrotherapy. The purpose of this study was to investigate the effect of hydrotherapy on communication skills of children with ASD. Method: This study is a single case study with A-B design. The statistical population includes all children with ASD in Alborz province in 2017. Purposeful sampling was used to select the sample. Accordingly, 3 children with autism disorder symptoms and symptoms were selected from Kahrizak rehabilitation center as a sample. The research instrument was the second version of the Gilliam Autism Rating Scale (GARS-2) and a researcher-made checklist for measuring communication skills. According to the Hollywick program, treatment sessions were conducted in 2 sessions (one hour) per week and for 2 months during 8 weeks (16 sessions). Results: During the visual analysis of chart, intervention was effective in all three subjects. The percentage of non-overlapping data (PND) in the two baseline and intervention situations for the 3 subjects were 100%, 80%, and 90%. Conclusion: The results showed that hydrotherapy significantly increased the communication skills of children with ASD, which could be achieved by enhancing sensory motor skills as well as providing the necessary contexts and conditions for communication. Therefore, it is recommended to use this method as a complementary approach in the field of education and rehabilitation of children with ASD along with other therapies.