Sleep-related problems (SRPs) are common and problematic among anxious youth but have not been investigated in anxious youth with autism spectrum disorder (ASD). Participants were 102 youth (ages 7–16 years) with ASD and comorbid anxiety. Youth and their primary caregiver were administered the Pediatric Anxiety Rating Scale. Parents completed the Multidimensional Anxiety Scale for Children–Parent (MASC-P) Report, Social Responsiveness Scale, and the Child Behavior Checklist (CBCL). A measure of SRPs was created from items from the CBCL and MASC-P. Results suggest SRPs were relatively common among youth with ASD and comorbid anxiety. The number of SRPs endorsed directly associated with parent ratings of social deficits, internalizing and externalizing symptoms, and anxiety symptoms, as well as with clinician-rated anxiety symptoms. Parent-rated internalizing symptoms predicted frequency of SRPs over and above social deficits, externalizing symptoms, and parent- and clinician-rated anxiety symptoms. A subset of 40 participants who completed family-based cognitive–behavioral therapy (CBT) experienced reduced SRPs following treatment. Implications, study limitations, and recommendations for future research are discussed.