BACKGROUND: Posttraumatic stress injury (PTSI) is a term used to describe a range of psychiatric difficulties which arise following exposure to a psychologically traumatic event. The impact of being diagnosed with multiple psychiatric conditions on the return-to-work (RTW) outcomes of individuals with PTSI has not been adequately researched. OBJECTIVE: The current study examined whether the presence of two or more psychiatric conditions occurring simultaneously is predictive of RTW outcomes in workers with PTSI. METHOD: A population-based cohort design was conducted using archival data from injured workers admitted to a PTSI rehabilitation program. Differences in RTW outcomes and demographic, administrative, and clinical variables were compared between individuals with single and multiple psychiatric diagnoses. A range of variables were entered into a multivariable logistic regression model predicting RTW. RESULTS: The final logistic regression model indicated workers had higher odds of RTW if they had a single psychiatric diagnosis (Adjusted Odds Ratio (AOR) 2.20), non-elevated scores on a measure of traumatic stress (AOR 1.85), and reported higher self-perceived readiness to RTW (AOR 1.24). CONCLUSION: Being diagnosed with multiple psychiatric conditions appears to be associated with more negative RTW outcomes following PTSI rehabilitation.