Abstract: Background: Surf therapy programs have demonstrated engagement among military samples, showing promising concurrent short-term reductions in symptoms of posttraumatic stress disorder, major depressive disorder, and generalised anxiety disorder; however, the long-term retained benefits of such programs have not been studied beyond three months. Methods: This nonrandomised controlled longitudinal study recruited current and former Australian Defence Force personnel (N = 116) to examine the effect of a ten-session surf therapy program (n = 88) compared to a control group (no intervention; n = 28). Validated self-assessment measures of post-traumatic stress, depression and anxiety symptoms were recorded at pre and post intervention, and at one month, four months, and seven months follow up. Results: Linear mixed model results reveal statistically significant post-program reductions in symptoms of posttraumatic stress (PCL-M; ss = -11.92, 95% CI [-17.44, -6.36]), depression (MDI; ss = -7.87, 95% CI [-12.35, -3.38]) and anxiety (GAD-7; ss = -4.02, 95% CI [-6.42, -1.57]), which were retained at 7-months follow-up. Clinically significant changes were also observed on all three outcomes. No changes were observed in the control group. Additional statistically significant beneficial effects for each model outcome were recorded for leisure surfing following the program. Most surf therapy participants continued surfing post program, indicating effective lifestyle change. Conclusions: This is the largest quantitative surf therapy study among adults to date, and the first to follow up seven months post program. It confirms previous surf therapy findings of high engagement and transdiagnostic effects, and provides new clinically relevant knowledge by demonstrating retained beneficial effects regardless of continued surfing post program.