Autism spectrum disorder is a neurodevelopmental condition characterized by social communication difficulties and restricted, repetitive behaviors. Autism spectrum disorder is highly co-morbid with other neuro-psychiatric disorders such as attention deficit hyperactivity disorder, anxiety, and depression. However, research on post-traumatic stress disorder among individuals with autism spectrum disorder is scarce. Furthermore, the understanding of shared mechanisms underlying autism spectrum disorder co-morbidity with other conditions may assist in both diagnostic and intervention efforts with affected individuals. This short report examined the role of brooding and reflective rumination as mediators between autism spectrum disorder and post-traumatic stress disorder. A total of 34 adults with autism spectrum disorder (with no intellectual impairment) and 66 typically developing controls, comparable on age and gender, filled out the PTSD Checklist for DSM-5 and the Rumination Response Scale. The results indicated increased post-traumatic stress disorder symptoms, as well as elevated brooding levels, in adults with autism spectrum disorder, compared to typically developing controls. Brooding, but not reflective rumination, mediated the association between autism spectrum disorder and post-traumatic stress disorder symptoms. Rumination and cognitive inflexibility, which are common in autism spectrum disorder, may exacerbate post-traumatic symptoms among traumatized individuals who have autism spectrum disorder. Interventions targeting brooding rumination and cognitive flexibility may assist in alleviating post-traumatic symptoms in individuals with autism spectrum disorder. Future studies should examine other psychological mechanisms which may underlie the autism spectrum disorder--post-traumatic stress disorder co-morbidity.