Autistic people appear to be at increased risk of post-traumatic stress disorder, and experiences involving violence or sexual abuse appear to be common in this population. However, knowledge concerning trauma in autistic people with co-occurring intellectual disabilities is limited. In this cross-sectional study, we explored the prevalence of post-traumatic stress disorder, as well as the prevalence and clinical correlates of reported experiences of violence and sexual abuse, in a clinical sample of 88 autistic adults with intellectual disabilities referred for mental health assessment. Only three participants (3.4%) were diagnosed with post-traumatic stress disorder, even if experiences involving violence (34.1%) or sexual abuse (17.0%) were common. Anxiety and affective disorders were common diagnoses among participants with these experiences. Controlling for autism characteristics, level of intellectual disability and communication skills, experiences of violence/sexual abuse were found to be associated with a measure of 'challenging' behaviours (Aberrant Behaviour Checklist). These results indicate that potentially traumatic experiences are common in autistic adults with intellectual disabilities referred for mental health assessment and that post-traumatic stress disorder may be under-recognised. The findings highlight the importance of trauma screening and post-traumatic stress disorder assessment, as well as the importance of trauma-informed care, in this population.