Introduction: autism spectrum disorder (ASD) and intellectual disability (ID) seem to influence the risk of and vulnerability to exposure to trauma and adverse events. While assessment of a psychiatric disorder in ASD and ID generally is challenging, identification of post-traumatic stress disorder (PTSD) seems particularly so, and knowledge does not seem easily accessible. Methods: This article provides a systematic review of studies describing trauma reactions in individuals with both ASD and ID, including studies involving any single case with the combination of ASD, ID, and PTSD. To systematically explore PTSD symptom presentation in the group, all reported symptoms from studies were assigned by DSM-5 criteria. Results: Eighteen studies met the inclusion criteria, eight group studies and 10 case studies. Assessment methodology in studies varied, as did the format of symptom report. DSM-5 criteria provided a useful framework for integrating findings across studies, indicating that PTSD may be identified in individuals with ASD and ID. However, symptoms involving alterations in arousal and negative alterations in thought and behavior seem more easily identified than symptoms of reexperiencing and avoidance. Conclusions: There is an urgent need to identify behavioral equivalents to PTSD symptoms in this group, making it possible to identify warning signs of trauma and abuse even if such incidents are not known to family or professional carers.