Co-occurring substance use disorder and posttraumatic stress disorder is prevalent among prisoners, however there are few evidence-based treatment options available for this population. This pilot study investigated the acceptability, feasibility and preliminary efficacy of implementing partial-dose (eight-session) Seeking Safety among 30 Australian male prisoners. Findings demonstrate that Seeking Safety is desirable, acceptable, and feasible among male prisoners with a history of substance use and trauma. In addition, the study indicated promising results for substance use and mental health outcomes, which is particularly notable given the overall severity of the sample. The study also had significant Aboriginal and/or Torres Strait Islander representation. A full scale trial of this intervention is required to further evaluate efficacy and inform practice to improve the mental health of this vulnerable population.