Research regarding psychometric properties of autism spectrum disorder questionnaires is lacking. This study explored the criterion validity of the ASRS 6–18 parent report (ASRS-6–18-P) in a large, well-characterized, real-world clinical sample of 422 children (\(\bar{X}_{\text{age}} = 10.04;\) autism [AUT] n = 139; non-autism [NOT] n = 283) evaluated with the Autism Diagnostic Observation Schedule-2, a gold-standard measure. Significant mean differences were observed for DSM-5, social, and unusual behaviors. Total, DSM-5, social, and unusual behaviors demonstrated significant correlations with ADOS-2 comparison scores (modules 1–3), but not with module 4 (raw) scores. DSM-5 and Unusual Behaviors demonstrated significant but poor AUCs (0.60). Findings with/without covariates (IQ/age) were overall similar. Sensitivity and specificity could not be optimized. The suggested cutpoint (T-score = 60) demonstrated unacceptably high false positive rates (> 76.33%). While findings suggest limited diagnostic utility of the ASRS-6–18-P, the sample’s complex psychiatric presentation and measurement error inherent in cutoff score application should be considered when generalizing results. Further research is recommended.