ImportanceThe treatment for extraperitoneal locally advanced rectal cancer (LARC) is neoadjuvant therapy (NAT) followed by total mesorectal excision (TME). Robust evidence on the optimal time interval between NAT completion and surgery is lacking.ObjectiveTo assess the association of time interval between NAT completion and TME with short- and long-term outcomes. It was hypothesized that longer intervals increase the pathologic complete response (pCR) rate without increasing perioperative morbidity.Design, Setting, and ParticipantsThis cohort study included patients with LARC from 6 referral centers who completed NAT and underwent TME between January 2005 and December 2020. The cohort was divided into 3 groups depending on the time interval between NAT completion and surgery: short (≤8 weeks), intermediate (>8 and ≤12 weeks), and long (>12 weeks). The median follow-up duration was 33 months. Data analyses were conducted from May 1, 2021, to May 31, 2022. The inverse probability of treatment weighting method was used to homogenize the analysis groups.ExposureLong-course chemoradiotherapy or short-course radiotherapy with delayed surgery.Main outcome and MeasuresThe primary outcome was pCR. Other histopathologic results, perioperative events, and survival outcomes constituted the secondary outcomes.ResultsAmong the 1506 patients, 908 were male (60.3%), and the median (IQR) age was 68.8 (59.4-76.5) years. The short-, intermediate-, and long-interval groups included 511 patients (33.9%), 797 patients (52.9%), and 198 patients (13.1%), respectively. The overall pCR was 17.2% (259 of 1506 patients; 95% CI, 15.4%-19.2%). When compared with the intermediate-interval group, no association was observed between time intervals and pCR in short-interval (odds ratio [OR], 0.74; 95% CI, 0.55-1.01) and long-interval (OR, 1.07; 95% CI, 0.73-1.61) groups. The long-interval group was significantly associated with lower risk of bad response (tumor regression grade [TRG] 2-3; OR, 0.47; 95% CI, 0.24-0.91), systemic recurrence (hazard ratio, 0.59; 95% CI, 0.36-0.96), higher conversion risk (OR, 3.14; 95% CI, 1.62-6.07), minor postoperative complications (OR, 1.43; 95% CI, 1.04-1.97), and incomplete mesorectum (OR, 1.89; 95% CI, 1.02-3.50) when compared with the intermediate-interval group.Conclusions and RelevanceTime intervals longer than 12 weeks were associated with improved TRG and systemic recurrence but may increase surgical complexity and minor morbidity.