Purpose:This study aimed to investigate the relationship between DNA damage response (DDR)-related protein expression and the clinical outcomes of patients with gastric cancer stage IV and recurrent advanced gastric cancer patients after gastrectomy treated with palliative first-line chemotherapy.Materials and Methods:A total of 611 gastric cancer patients underwent D2 radical gastrectomy at Chung-Ang University Hospital between January 2005 and December 2017, of which 72 patients who received gastrectomy treatment with palliative chemotherapy were enrolled in this study. We performed the immunohistochemical assessment of MutL Homolog 1 (MLH1), MutS Homolog 2 (MSH2), at-rich interaction domain 1 (ARID1A), poly adenosine diphosphate-ribose polymerase 1 (PARP-1), breast cancer susceptibility gene 1 (BRCA1), and ataxia-telangiectasia mutated (ATM) using formalin-fixed paraffin-embedded samples. In addition, Kaplan-Meier survival analysis and Cox regression models were used to evaluate independent predictors of overall survival (OS) and progression-free survival (PFS).Results:Among the 72 patients studied, immunohistochemical staining analysis indicated deficient DNA mismatch repair (dMMR) in 19.4% of patients (n = 14). The most common DDR gene with suppressed expression was PARP-1 (n = 41, 56.9%), followed by ATM (n = 26, 36.1%), ARID1A (n = 10, 13.9%), MLH1 (n = 12, 16.7%), BRCA1 (n = 11, 15.3%), and MSH2 (n = 3, 4.2%). HER2 (n = 6, 8.3%) and PD-L1 (n = 3, 4.2%) were expressed in 72 patients. The dMMR group exhibited a significantly longer median OS than the MMR proficient (pMMR) group (19.9 months vs. 11.0 months; hazard ratio [HR] 0.474, 95% confidence interval [CI] = 0.239-0.937, P = 0.032). The dMMR group exhibited a significantly longer median PFS than the pMMR group (7.0 months vs. 5.1 months; HR= 0.498, 95% CI = 0.267-0.928, P = 0.028).Conclusions:Of stage IV gastric cancer and recurrent gastric cancer patients who underwent gastrectomy, the dMMR group had a better survival rate than the pMMR group. Although dMMR is a predictive factor for immunotherapy in advanced gastric cancer, further studies are needed to determine whether it is a prognostic factor for gastric cancer patients treated with palliative cytotoxic chemotherapy.