In the present study, we studied the expression of T-bet, a key marker for type 1 immune responses, within the tumor microenvironment of gastric cancer, and analyzed its association with clinicopathological parameters. One hundred and fifty-two archival paraffin-embedded gastric tumor tissues were collected, and the expression of T-bet in these cancer tissue specimens was examined by immunohistochemistry. T-bet(+) tumor-infiltrating lymphocytes (TILs) in some gastric cancer tissues were further characterized by flow cytometric analysis. The density of T-bet(+) TILs in gastric cancer tissues in relation to patient's clinicopathological parameters and postoperative prognosis has been analyzed. Herein, we have found significant increases in T-bet(+) lymphocytes in tumor tissues as compared with normal stomach tissues, gastritis tissues or gastric polyp specimens. T-bet(+) cells mainly consisted of CD4(+), CD8(+) and CD56(+) TILs. In addition, lower numbers of T-bet(+) TILs were associated with poor clinicopathological parameters such as invasion to muscular layer, larger tumor size and advanced cancer stages. Moreover, patients with higher numbers of T-bet(+) TILs have longer disease-free survival and overall survival. Thus, our study supports the idea that tumor growth elicits spontaneous type 1 cellular immune responses and tumor progression is associated with suppression of antitumor immunity. T-bet expression within tumor can serve as a prognostic indicator for gastric cancer and a potential biomarker for immunotherapy.