Background: Recently, the negative impact of postoperative complications on long-term survival outcomes has been reported in patients with gastric cancer. However, most are single center, retrospective studies with different definitions of postoperative complications. The objective of this study was to evaluate the impact of postoperative complications on long-term outcomes using the data of a multicenter randomized controlled trial (JCOG1001). Methods: This study included 1191 out of all 1204 patients enrolled in JCOG1001 which was aimed to confirm the superiority of bursectomy for patients with cT3/4a locally advanced gastric cancer. Complications were graded by Clavien-Dindo classification. The relationships between the grade (≥grade II or ≥grade III) or type (all or intraabdominal infectious (pancreatic fistula, anastomotic leakage, and intra-abdominal abscess.)) of complications and survival outcomes were evaluated. Results: The incidences of ≥grade II and ≥grade III all complications were 23.0% and 9.7%, and those of ≥grade II and ≥grade III intraabdominal infectious complications were 13.4% and 6.9%, respectively. The hazard ratios for overall survival (OS) of patients with ≥grade II and ≥grade III all complications and those of patients with ≥grade II and ≥grade III intraabdominal infectious complications were shown in Table. With whichever definition we adopted, postoperative complications were significantly associated with OS in both univariable and multivariable analysis. Conclusions: Postoperative complication was identified as an independent prognostic factor in patients with cT3/4a gastric cancer. Hazard ratios for overall survival by univariable and multivariable Cox proportional hazard model. Clinical trial information: UMIN000003688.