Insomnia is one of the most common co-occurring disorders in children with autism spectrum disorder (ASD). The purpose of this cross-sectional cohort study of 270 children with ASD was to assess the prevalence of insomnia using the Pediatric Sleep Clinical Global Impression Scale and to correlate sleep problems with cognitive level, adaptive behavior, parental stress, behavioral and emotional problems, and severity of core symptoms. Our findings revealed that one-third of children had insomnia and were related to reduced adaptive behaviors and higher levels of parental stress and behavioral and emotional problems. Insomnia must be evaluated in children with ASD to define an appropriate intervention to potentially alleviate the severity of clinical correlates and parental burden.