Gastric cancer is the fourth most common cancer worldwide, with a low 5-year survival rate. Epigenetic modification plays pivotal roles in gastric cancer development. However, the role of histone-modifying enzymes in gastric cancer remains largely unknown. Here we report that Sirt7, a NAD(+)-dependent class III histone deacetylase, is over-expressed in human gastric cancer tissues. Sirt7 level is significantly correlated with disease stage, metastasis, and survival. Knockdown of Sirt7 in gastric cancer cells inhibits cell proliferation and colony formation in vitro. In vivo subcutaneous xenograft results also show that Sirt7 knockdown can markedly repress gastric cancer cell growth. In addition, Sirt7 depletion induces apoptosis in gastric cancer cells via up-regulating expression of pro-apoptotic proteins and down-regulating anti-apoptotic proteins. Mechanically, Sirt7 binds to the promoter of miR-34a and deacetylases the H3K18ac, thus represses miR-34a expression. Reversely, depletion of miR-34a inhibits gastric cancer apoptosis induced by Sirt7 knockdown, and restores cellular capacity of proliferation and colony formation. miR-34a depletion reduces Sirt7-knockdown-induced arrest of gastric growth. Finally, miR-34a is tightly associated with survival of patients with gastric cancer.