Background: Urologists practicing in single-specialty groups with ownership in radiation vaults are more likely to treat men with prostate cancer. The effect of urology practice divestment in radiation vault ownership on treatment patterns of men with prostate cancer is unclear. Methods: A 20% sample of national Medicare claims was used to perform a retrospective cohort study of men with prostate cancer, diagnosed between 2010 and 2019, managed by single specialty urology groups. All men were followed through December 31, 2020. Urology groups were categorized by radiation vault ownership for each year as nonowners, owners, and divested owners. The primary outcome was use of treatment and the secondary outcome was use of intensity-modulated radiation therapy (IMRT). Both outcomes were assessed for the 12-month period after diagnosis. Using a difference-in-differences design, multivariable logistic regression was used to measure the effect of radiation vault divestment on outcomes. Subgroup analyses assessed outcomes by non-cancer mortality risk (high (> 50%) vs. low (≤ 50%)) within 10 years of diagnosis. Results: Among 72 urology practices that owned radiation vaults, six divested during the study. Divestment led to a decrease in treatment compared with those managed at continuously owning practices (difference-in-differences estimate, -13%, p=0.03). The use of IMRT also decreased, but this change was not statistically significant (difference-in-differences estimate, -8%, p=0.21). In men with a high non-cancer mortality risk, treatment (difference-in-differences estimate, -28%, p<0.001) and use of IMRT (difference-in-differences estimate, -27%, p<0.001) decreased after divestment. Conclusions: Urology single specialty group divestment in radiation vault ownership leads to a decrease in prostate cancer treatment. This decrease was most pronounced in newly diagnosed men that had a high non-cancer mortality risk. This has important implications for healthcare reform, suggesting that programs that can incentive constraints on utilization may be effective in reducing prostate cancer overtreatment.