There is significant heterogeneity amongst individuals with Autism Spectrum Disorder (ASD) in symptom presentation and severity. An understanding of the factors that contribute to and modulate symptom severity are critical to informing prognosis, stratification, and treatment decisions. Maternal prenatal stress exposure is a nonspecific risk factor for a wide array of neurodevelopmental outcomes in subsequent offspring. Emerging evidence suggests that prenatal maternal stress may increase ASD risk and contribute to variability in autism‐like traits in the general population. In the current study, we aimed to determine whether prenatal maternal exposure to stressful life events is associated with symptom severity amongst individuals with ASD. We performed multiple regression analyses to examine associations between retrospectively recalled maternal prenatal stressful life events and the severity of ASD‐associated symptoms in 174 children with ASD (Mage = 9.09 years; SD = 3.81). ASD‐related symptom severity was measured using the Social Responsiveness Scale and communication abilities were measured using the Children's Communication Checklist. Exposure to prenatal stressful life events was a significant predictor of ASD‐related symptom severity (t = 2.014; P = .048) and communication abilities (t = −2.925; P = .004) amongst children with ASD, even after controlling for a range of sociodemographic and obstetric variables. Follow‐up analyses demonstrated significant increases in symptom severity only in the context of multiple (two or more) prenatal stressful life events. Together, these findings indicate that ASD, in the context of prenatal maternal stress exposure, may be associated with a more severe phenotype, particularly when there are multiple prenatal exposures. Autism Res 2017. © 2017 International Society for Autism Research, Wiley Periodicals, Inc.