Parents of children with autism spectrum disorder (ASD) report higher levels of stress than do parents of typically developing (TD) children and children with other developmental disabilities (DD). Parenting stress has been linked to low quality of life and is thought to affect the parent-child relationship. Throughout the diagnostic process, both parents and clinicians typically act as informants or raters of a child’s symptoms and behaviors. Research indicates that parents’ perceptions and understanding of their child’s symptoms may be influenced by their stress levels. Therefore, because parents’ stress may influence their report of their child’s characteristics, it is essential to understand if and how parenting stress impacts the level of agreement between clinician and parent ratings, particularly when assessing for ASD symptom severity. This study examined the relation between parent- and clinician-rated ASD symptom severity as a function of parenting stress in a sample of children with and without ASD. Additionally, this study examined the potential influence of child restrictive and repetitive behaviors (RRBs) and age on the relation between raters of ASD symptom severity. Results indicated no relation between parent- and clinician-rated ASD symptom severity and a negative relation between parenting stress and clinician-rated ASD symptoms. Interactions between predictor variables were also examined. Overall, results demonstrate the complexity of parents’ stress levels and their perception of their child’s symptom severity that might play a role in the relation between raters. Future research should aim to shed more light on parental factors other than stress, such as parent knowledge, that may play a role in the diagnostic process.