Objective: To evaluate the benefit of adjuvant radiotherapy (RT) after surgical resection for extrahepatic bile duct (EHBD) cancer. Methods: From 1997 to 2015, 59 patients with EHBD cancer were the subject of this study; 36 patients not undergoing adjuvant treatment after surgery (observation group) and 23 patients receiving adjuvant RT (RT group) were compared. Microscopic residual disease (R1) was in 9 (25%) patients and 5 (22%) patients, and macroscopic residual disease (R2) was in 2 (6%) patients and 6 (26%) patients in the observation and RT groups, respectively. Adjuvant RT was delivered to the tumour bed and regional lymph nodes up to 50.4Gy (range, 45-61Gy). Results: With a median follow-up of 19 months, local recurrence was observed in 10 (28%) patients and 2 (9%) patients in the observation and RT groups, respectively. On univariate analysis, the 5-year local recurrence-free survival (LRFS) rates were 50% in the observation group and 54% in the RT group (p=0.401). The 5-year overall survival (OS) rates were 29.3% in the observation group and 26.3% in the RT group (p=0.602). On multivariable analysis, however, adjuvant RT significantly improved LRFS [hazard ratio (HR), 0.310; 95% confidence interval (CI), 0.100-0.963; p=0.043] and had a trend towards increased OS (HR, 0.491; 95% CI, 0.219-1.102; p=0.085). Resection margin (RM) status was also correlated with LRFS (HR for R1 6.134, 95% CI 2.051-18.344; and HR for R2 18.551, 95% CI 3.680-93.520; p<0.001) and OS (HR for R1 1.816, 95% CI 0.853-3.867; and HR for R2 3.564, 95% CI 1.175-10.809; p=0.054). Conclusion: RM status was a significant prognosticator of EHBD cancer, and adjuvant RT improved local control rate; thereby, survival rate might be increased. Advances in knowledge: The benefit of adjuvant RT in EHBD cancer was demonstrated via comparison with observation group.