IMPORTANCE: Postoperative radiotherapy remains underused for men with biochemical recurrence or adverse pathological features after radical prostatectomy (RP). Stereotactic body radiotherapy (SBRT) may improve utilization and poses potential radiobiological advantages. OBJECTIVE: To evaluate physician-reported late toxic effects and 2-year patient-reported outcomes (PROs) following post-RP SBRT. DESIGN, SETTING, AND PARTICIPANTS: This phase 2, single-arm trial was conducted in 2 academic centers in the US and included a comparator cohort. Men with post-RP prostate-specific antigen greater than 0.03 ng/mL or adverse pathologic features were included. Data were collected from February 2018 to March 2021, and data were analyzed from January to October 2024. INTERVENTIONS: SBRT delivered at 30 to 34 Gy in 5 fractions to the prostate bed. Nodal irradiation, boost to gross disease, and/or hormonal therapy were delivered per physician discretion. MAIN OUTCOMES AND MEASURES: Late toxic effects (more than 90 days after treatment) were graded according to Common Terminology Criteria for Adverse Events version 4.03. PROs were measured using Expanded Prostate Cancer Index-26. The proportion of men whose PROs had decrements greater than twice the threshold for minimal clinically important difference (MCID) at any point during the first 2 years were evaluated. The longitudinal PROs for men receiving SBRT was compared with a cohort of 200 men receiving postoperative conventionally fractionated radiotherapy (CFRT) using logistic regression, while adjusting for baseline scores, age, and receipt of nodal irradiation. RESULTS: Of 100 patients treated with post-RP SBRT, the median (IQR) age was 68.5 (63.9-71.4) years, and the median (IQR) follow-up was 43 (37-53) months. Cumulative incidence of late grade 2 and 3 genitourinary toxic effects was 25% and 4%, respectively, and of late grade 2 and 3 gastrointestinal tract toxic effects was 3% and 3%, respectively. The proportion of patients with decrements more than 2-fold the MCID in PROs was 38.9% (37 of 95) for urinary incontinence, 17.9% (17 of 95) for urinary irritation, and 34.1% (31 of 91) for bowel function. Compared with the CFRT cohort, the adjusted odds ratio for patients receiving SBRT experiencing decrements more than 2-fold the MCID was 1.55 (95% CI, 0.87-2.76; P = .14) for urinary incontinence, 0.94 (95% CI, 0.46-1.94; P = .87) for urinary irritation, and 1.03 (95% CI, 0.57-1.84; P = .93) for bowel function. CONCLUSIONS AND RELEVANCE: In this nonrandomized clinical trial, post-RP SBRT was well-tolerated, with no measurably different decline in urinary or bowel PROs through 2 years compared with CFRT. Randomized studies and longer follow-up will better define the toxic effects and efficacy profile of post-RP SBRT. TRIAL REGISTRATION: ClinicalTrials.gov Identifier: NCT03541850.