Compared to the social communication domain, considerably less is known about the cause, development, and impact of restricted, repetitive behaviors interests and activities in children with autism spectrum disorder, including possible sex differences. This study examined sex differences in clinically identified (Autism Diagnostic Observation Schedule) restricted and repetitive behavior symptoms using the largest known sample (N = 1024) of age-matched and intelligence quotient-matched female and male children with autism spectrum disorder. More similarities than differences were observed; however, younger higher functioning and older lower functioning females presented reduced rates on the Autism Diagnostic Observation Schedule restricted and repetitive behavior subcategory "unusually repetitive/excessive, stereotyped behaviors" compared to similar males. These findings identify key restricted and repetitive behavior similarities and differences among young females and males with autism spectrum disorder and emphasize the need for a deeper understanding of the female autism phenotype.