Objective: Psychiatric service dogs are an emerging complementary treatment for military members and veterans with posttraumatic stress disorder (PTSD). Yet despite anecdotal accounts of their value, there is a lack of empirical research on their efficacy. The current proof-of-concept study assessed the effects of this practice. Method: A nonrandomized efficacy trial was conducted with 141 post-9/11 military members and veterans with PTSD to compare usual care alone (n = 66) with usual care plus a trained service dog (n = 75). The primary outcome was longitudinal change on The PTSD Checklist (PCL; Weathers, Litz, Herman, Huska, & Keane, 1993), including data points from a cross-sectional assessment and a longitudinal record review. Secondary outcomes included cross-sectional differences in depression, quality of life, and social and work functioning. Results: Mixed-model analyses revealed clinically significant reductions in PTSD symptoms from baseline following the receipt of a service dog, but not while receiving usual care alone. Though clinically meaningful, average reductions were not below the diagnostic cutoff on the PCL. Regression analyses revealed significant differences with medium to large effect sizes among those with service dogs compared with those on the waitlist, including lower depression, higher quality of life, and higher social functioning. There were no differences in employment status, but there was lower absenteeism because of health among those who were employed. Conclusion: The addition of trained service dogs to usual care may confer clinically meaningful improvements in PTSD symptomology for military members and veterans with PTSD, though it does not appear to be associated with a loss of diagnosis.