Purpose: The role of adjuvant radiotherapy (aRT) in treating patients with pN1 prostate cancer is controversial. We tested the hypothesis that the impact of aRT on cancer-specific mortality (CSM) in these individuals is related to tumor characteristics. Methods: We evaluated 1,107 patients with pN1 prostate cancer treated with radical prostatectomy and anatomically extended pelvic lymph node dissection between 1988 and 2010 at two tertiary care centers. All patients received adjuvant hormonal therapy with or without aRT. Regression tree analysis stratified patients into risk groups on the basis of their tumor characteristics and the corresponding CSM rate. Cox regression analysis tested the relationship between aRT and CSM rate, as well as overall mortality (OM) rate in each risk group separately. Results: Overall, 35% of patients received aRT. At multivariable analysis, aRT was associated with more favorable CSM rate (hazard ratio [HR], 0.37; P < .001). However, when patients were stratified into risk groups, only two groups of men benefited from aRT: (1) patients with positive lymph node (PLN) count ≤ 2, Gleason score 7 to 10, pT3b/pT4 stage, or positive surgical margins (HR, 0.30; P = .002); and (2) patients with PLN count of 3 to 4 (HR, 0.21; P = .02), regardless of other tumor characteristics. These results were confirmed when OM was examined as an end point. Conclusion: The beneficial impact of aRT on survival in patients with pN1 prostate cancer is highly influenced by tumor characteristics. Men with low-volume nodal disease (≤ two PLNs) in the presence of intermediate- to high-grade, non–specimen-confined disease and those with intermediate-volume nodal disease (three to four PLNs) represent the ideal candidates for aRT after surgery.