Accurate representation of autism spectrum disorder (ASD) in fragile X syndrome (FXS) is necessary for the field. We examined classifications of ASD using three approaches—Autism Diagnostic Observation Schedule (ADOS-2; Lord et al. 2012), Childhood Autism Rating Scale (CARS2-ST; Schopler et al. 2010), and Vineland Adaptive Behavior Scales (VABS-II; Sparrow et al. 2005)—in 45 adolescents with FXS. Maladaptive items from the VABS-II plus a maternal interview were matched with Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5; American Psychiatric Association 2013) criteria for ASD. Results indicated discrepant classifications. The ADOS-2 yielded the highest rate of comorbid autism (71%); CARS2-ST and VABS-II/DSM-5 yielded lower rates (38% and 42%, respectively). A singular measure of autism symptomology is insufficient to characterize autism in FXS.