Trefoil factor 1 (TFF1) is a small cysteine-rich secreted protein which is principally expressed in the superficial cells of gastric mucosa. In gastric cancer, TFF1 is downregulated and plays an important role. Gastrokine 1 (GKN1) is a secreted protein with similar expression and biological functions to TFF1. This study aimed to determine the expression and biological functions of TFF1 and the relationships between TFF1 and GKN1 in gastric cancer. RT-PCR and immunohistochemistry were performed to detect TFF1 expression in gastric cancer cell lines and tissues. The transfected and co-transfected AGS cells which stably expressed TFF1 or both TFF1 and GKN1 were generated. Phenotypic changes such as cell viability, apoptosis and cell cycle modulation were assayed in the transfected cells. We found that TFF1 expression was significantly downregulated or lost in gastric cancer cell lines, gastric dysplasia and cancer. Restoration of TFF1 expression in AGS cells suppressed tumor cell viability and arrested AGS cells in the G1-S transition phase after olomoucine treatment. However, TFF1 was unable to induce cell apoptosis. In co-transfected cells, we found that TFF1 and GKN1 did not directly interact at the protein level. GKN1 was unable to cooperate with TFF1 on cell viability suppression, cell apoptosis and differentiation. Together, these results indicate that TFF1 expression is significantly downregulated in gastric cancer. TFF1 inhibited cell proliferation by delaying G1-S phase transition but not by inducing apoptosis. TFF1 may not interact or cooperate with GKN1 at the protein and functional level.