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 50.4 Gy (range, 45-61 Gy). 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). CONCLUSIONS 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.