BACKGROUND: Mental health problems are common amongst adults with an Autism Spectrum Disorder (ASD). Stressful and traumatic life events can trigger or exacerbate symptoms of anxiety, depression and PTSD. In the general population, transdiagnostic processes such as suppression and perseverative thinking are associated with responses to trauma and mental health symptoms. AIMS: This study explored the relationships between thought suppression, perseverative thinking and symptoms of depression, anxiety and PTSD in ASD adults who reported exposure to a range of DSM-5 and non-DSM-5 traumatic events. METHODS: 59 ASD adults completed a series of online self-report questionnaires measuring trauma, transdiagnostic cognitive processes, and mental health symptoms. RESULTS: Probable PTSD rarely occurred in isolation and was associated with depression and anxiety symptoms in trauma-exposed ASD adults. All cognitive processes and mental health symptoms were positively associated with one another, regardless of whether the trauma met DSM-5 PTSD Criterion A. When accounting for both cognitive processes, only thought suppression significantly predicted PTSD and anxiety symptoms, while only perseverative thinking significantly predicted depression symptoms. CONCLUSIONS AND IMPLICATIONS: These preliminary results suggest that different cognitive processes more strongly affect anxiety/PTSD versus depression symptom severity in trauma-exposed ASD adults, although co-occurring symptoms are common. Implications for assessment, treatment and future research are discussed.