Purpose To retrospectively evaluate the role of adjuvant radiotherapy (ART) as monotherapy in a cohort of prostate cancer patients with undetectable prostate-specific antigen (PSA) after surgery and to propose a risk stratification system. Methods Between 2003 and 2010, 174 consecutive patients were treated with ART (median dose 71 Gy) at a single institution. Subsequently, we assigned a score of 1 to the following risk factors (RF): T stage ≥3b, presurgical PSA ≥10 ng/mL, pathologic Gleason score (GS) ≥4 + 3, and positive surgical margin (R1). The scores were then summed to stratify the population into low risk (LR), intermediate risk (IR), and high risk (HR). Results Median follow-up was 61 months (range 4-105). Five-year biochemical relapse-free survival (bRFS), clinical relapse-free survival (cRFS), and overall survival (OS) were respectively 93%, 97.1%, and 98.6%. On univariate analyses, GS was the only variable related to bRFS (p = 0.04) and to cRFS (p = 0.05). Any variable was related to OS. Kaplan-Meier analysis showed that HR patients (3-4 RF) had a worse bRFS (p = 0.02) compared to LR patients (0 RF or R1 as single RF); IR patients (1-2 RF) had a lower bRFS compared to LR patients (p = 0.06). Patients with R1 as single RF have the same bRFS as patients with 0 RF (p = 0.6) and are considered as LR patients. Conclusions Adjuvant radiotherapy leads to excellent bRFS and cRFS rates at 5 years (93.3% and 97.1%, respectively) in our population. Patients with multiple RF are at higher risk of bRFS. Patients with R1 as single RF have bRFS rates comparable to patients without any RF.