The present investigation details the assessment and use of Behavioral Activation (BA) therapy to treat a 37-year-old male police officer/military veteran suffering from posttraumatic stress disorder (PTSD) and major depressive disorder (MDD). This case study is an attempt to expand empirical knowledge regarding BA, comorbid PTSD and MDD, and treatment outcome research specifically relevant to these comorbid diagnoses. The BA treatment consisted of 11 sessions, which occurred on a weekly basis. Self-report data were gathered at each session and again at midpoint between each session. At posttreatment assessment, self-report and observer-rated data indicated that the client no longer met criteria for either PTSD or MDD. Results at 1-month follow-up suggested that the therapeutic gains were not only maintained, but that the client continued to improve. It is argued that BA may be an effective treatment for comorbid PTSD and MDD and the theoretical rationale is provided.