A number of international studies reveal high prevalence rates of posttraumatic stress disorder (PTSD) within homeless populations. Recent research on PTSD indicates that cognitive responses to trauma are critical in determining who develops the disorder. In Australia, PTSD has not been investigated among homeless adults. We aimed to improve understanding of PTSD and related cognitions in this cohort. Seventy homeless people aged 18 to 73 were randomly sampled and interviewed. The majority reported at least one traumatic event in their lifetime (98.2%) and 78.6% met criteria for a lifetime diagnosis of PTSD. In 70.5% of cases, PTSD onset preceded or coincided with the first homeless episode. It was found that those with a current diagnosis of PTSD scored significantly higher overall on measures of posttraumatic cognitions and early maladaptive schemas (or beliefs). In particular, this group scored higher on schemas that centre on the world being entirely dangerous and the self being totally inept. A mediational analysis showed that when trauma variables were controlled, the relationship between early maladaptive schemas and PTSD symptom severity was mediated by posttraumatic cognitions. These findings have implications for public policy on homelessness and mental health, homeless service provision, PTSD theory, and PTSD treatment for homeless adults.