Background: Postoperative complications (POC) are associated with poor oncologic outcomes in gastric cancer. We sought to evaluate the impact of POC on survival in patients with gastric cancer treated with upfront surgery (UpSurg) versus those treated with preoperative therapy (PreT). Methods: We analyzed data from a prospectively maintained database of patients who had undergone resection of their gastric cancer at our institution. Patients with T1N0 or M1 lesions, recurrent disease, and mortality within 90 days were excluded. Survival was compared between patients with and without POC in the UpSurg and PreT groups. Cox regression analyses were used to examine factors associated with overall survival (OS) and disease-free survival (DFS). Results: A total of 421 patients underwent resection of gastric cancer: 30% underwent upfront surgery, and 51% had a POC. Among patients who had POCs, 71% were infectious and 53% were Clavien–Dindo grade III or IV. UpSurg patients with a POC had shorter OS (5-year, 47 vs. 85%; p < 0.001) and DFS (5-year, 46 vs. 76%; p < 0.001) than those without a POC. In contrast, there was no difference in OS (5-year, 57 vs. 63%; p = 0.77) and DFS (5-year, 52 vs. 52%; p = 0.52) between PreT patients with and without POC. Multivariable Cox regression model demonstrated that a POC in UpSurg patients had significant impact on DFS (2.6 [95% confidence interval (CI) 1.48–4.74]), whereas it did not in PreT patients (0.9 [95% CI 0.70–1.33]). Conclusions: The use of preoperative therapy negated the impact of POCs on OS and DFS in patients undergoing resection for gastric cancer.