Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by restricted, repetitive patterns of behavior and interests (RRBIs). With the latest update to the Diagnostic and Statistical Manual of Mental Disorders, a severity level rating is assigned to the two core features of ASD (American Psychiatric Association in Diagnostic and statistical manual of mental disorders 5 American Psychiatric Association Washington, D.C., 2013). Previous studies have identified factors associated with RRBI severity; however, the relationship among RRBIs, adaptive functioning, and diagnostic severity level remains unclear. The present study investigated whether adaptive functioning and parent-reported ASD symptoms predict RRBI severity in young children with ASD. Additionally, a fine-grained analysis was conducted to examine the factors associated with diagnostic severity level ratings. Several significant associations were found. Study findings and implications for assessment and treatment of RRBIs are discussed.