IMPORTANCE: Treatment-related regret is an integrative, patient-centered measure that accounts for morbidity, oncologic outcomes, and anxiety associated with prostate cancer diagnosis and treatment. OBJECTIVE: To assess the association between treatment approach, functional outcomes, and patient expectations and treatment-related regret among patients with localized prostate cancer. DESIGN, SETTING, AND PARTICIPANTS: This population-based, prospective cohort study used 5 Surveillance, Epidemiology, and End Results (SEER)-based registries in the Comparative Effectiveness Analysis of Surgery and Radiation cohort. Participants included men with clinically localized prostate cancer from January 1, 2011, to December 31, 2012. Data were analyzed from August 2, 2020, to March 1, 2021. EXPOSURES: Prostate cancer treatments included surgery, radiotherapy, and active surveillance. MAIN OUTCOMES AND MEASURES: Patient-reported treatment-related regret using validated metrics. Regression models were adjusted for demographic and clinicopathologic characteristics, treatment approach, and patient-reported functional outcomes. RESULTS: Among the 2072 men included in the analysis (median age, 64 [IQR, 59-69] years), treatment-related regret at 5 years after diagnosis was reported in 183 patients (16%) undergoing surgery, 76 (11%) undergoing radiotherapy, and 20 (7%) undergoing active surveillance. Compared with active surveillance and adjusting for baseline differences, active treatment was associated with an increased likelihood of regret for those undergoing surgery (adjusted odds ratio [aOR], 2.40 [95% CI, 1.44-4.01]) but not radiotherapy (aOR, 1.53 [95% CI, 0.88-2.66]). When mediation by patient-reported functional outcomes was considered, treatment modality was not independently associated with regret. Sexual dysfunction, but not other patient-reported functional outcomes, was significantly associated with regret (aOR for change in sexual function from baseline, 0.65 [95% CI, 0.52-0.81]). Subjective patient-perceived treatment efficacy (aOR, 5.40 [95% CI, 2.15-13.56]) and adverse effects (aOR, 5.83 [95% CI, 3.97-8.58]), compared with patient expectations before treatment, were associated with treatment-related regret. Other patient characteristics at the time of treatment decision-making, including participatory decision-making tool scores (aOR, 0.80 [95% CI, 0.69-0.92]), social support (aOR, 0.78 [95% CI, 0.67-0.90]), and age (aOR, 0.78 [95% CI, 0.62-0.97]), were significantly associated with regret. Results were comparable when assessing regret at 3 years rather than 5 years. CONCLUSIONS AND RELEVANCE: The findings of this cohort study suggest that more than 1 in 10 patients with localized prostate cancer experience treatment-related regret. The rates of regret appear to differ between treatment approaches in a manner that is mediated by functional outcomes and patient expectations. Treatment preparedness that focuses on expectations and treatment toxicity and is delivered in the context of shared decision-making should be the subject of future research to examine whether it can reduce regret.