<b><i>Objectives:</i></b> The aims of this study were to assess expressions of the DNA damage response (DDR)-related proteins and to investigate their clinical significances in gastric carcinoma. <b><i>Methods:</i></b> Two independent cohorts, a training set (n = 524) and validation set (n = 394), of gastric cancer patients were enrolled. Ataxia telangiectasia mutated (ATM), checkpoint kinase 2 (Chk2), and p53 expressions were examined by immunohistochemistry using tissue microarray. <b><i>Results:</i></b> ATM loss, Chk2 loss, and p53 positivity were observed in 21.8, 14.1, and 36.1% of the training set, and in 17.3, 12.2, and 35.8% of the validation set, respectively. In the training set, the aberrant expressions of ATM, Chk2, or p53 were significantly associated with an advanced TNM stage and poor disease-specific survival. This association was verified in the validation set. Chk2 positivity and p53 negativity were significantly related to a prolonged disease-specific survival. Also, patients with nonaberrant expressional levels of all 3 DDR-related proteins had a more favorable outcome than others. Multivariate analyses showed that Chk2 loss and at least 1 aberrant DDR-related protein remained as independent prognostic factors of poor disease-specific survival. <b><i>Conclusions:</i></b> This study elucidated the prognostic implications of DDR-related proteins, and suggests that their aberrant expressions play critical roles in the development and progression of gastric cancer.