Challenges with identifying and measuring anxiety in children and young people with an Autism Spectrum Disorder (ASD) have prompted studies examining the reliability of and agreement between different informants. In this study, agreement and factors influencing agreement (caregivers’ educational level and stress; child’s age, gender, verbal and performance IQ) between parent and child reports of anxiety symptoms was examined in a sample of 70 children with an ASD (66 boys; 9–16 years; mean age = 11.21, SD = 1.79 years). The participants completed the Spence Children’s Anxiety Scale (SCAS) - Child Version, while their parents completed the SCAS - Parent Version and the Parenting Stress Index. Children rated themselves as having significantly more anxiety symptoms compared to parental ratings of children’s anxiety. Parent and child reports of anxiety were significantly positively correlated for separation, social and generalized anxiety and for total anxiety scores with mostly medium effect sizes, but not for panic attack or obsessive-compulsive subscale scores. Higher parent-child agreement was found for anxiety symptoms associated with clearly observable behaviours. More agreement was associated with higher child verbal IQ and lower levels of parenting stress. Specifically, increased parental stress was associated with more discrepant caregiver-child reports of social anxiety symptoms. Our findings support the need for multi-informant data in order to capture a more comprehensive clinical picture of anxiety symptoms in children with an ASD and to consider the informants’ own stress and anxiety levels when obtaining caregivers’ perspectives.