Aim The aim was to compare the pathological complete response (pCR) rate at 8 compared to 12 weeks' interval between completion of neoadjuvant chemoradiotherapy (CRT) and surgery in patients with locally advanced rectal cancer. Method This was a randomized trial which included a total of 330 patients from two institutions. Patients with locally advanced (T3-4N0M0, TxN+M0) rectal cancer were randomized into 8- and 12-week interval groups. All the patients received long-course CRT (45 Gy in 1.8 Gy fractions and concomitant oral capecitabine or 5-fluorouracil infusion). Surgery was performed at either 8 or 12 weeks after CRT. The primary end-point was pCR. Secondary end-points were sphincter preservation, postoperative morbidity and mortality. Results Two-hundred and fifty-two patients (n = 125 in the 8-week group, n = 127 in the 12-week group) were included. Demographic and clinical characteristics were similar between groups. The overall pCR rate was 17.9% (n = 45): 12% (n = 15) in the 8-week group and 23.6% (n = 30) in the 12-week group (P = 0.021). Sphincter-preserving surgery was performed in 107 (85.6%) patients which was significantly higher than the 94 (74%) patients in the 12-week group (P = 0.016). Postoperative mortality was seen in three (1.2%) patients overall and was not different between groups (1.6% in 8 weeks vs 0.8% in 12 weeks, P = 0.494). Groups were similar in anastomotic leak (10.8% in 8 weeks vs 4.5% in 12 weeks, P = 0.088) and morbidity (30.4% in 8 weeks and 20.1% in 12 weeks, P = 0.083). Conclusion Extending the interval between CRT and surgery from 8 to 12 weeks resulted in a 2-fold increase in pCR rate without any difference in mortality and morbidity.