Aberrant expression and function of microRNAs (miRNAs) play a critical role in the development and progression of various human cancers including gastric cancer. However, the clinical significance and underlying mechanisms of miR-340 remain largely unknown in gastric cancer. In the present study, we demonstrated that the expression of miR-340 was aberrantly elevated in both gastric cancer tissues and cells. Moreover clinical association analyses disclosed that the elevated level of miR-340 was significantly associated with unfavorable clinicopathological characteristics of the gastric cancer patients, such as poor differentiation, large tumor size and advanced tumor-node-metastasis (TNM) stage. Gastric cancer patients with high expression of miR-340 had prominently shorter overall survival and disease-free survival. Functionally, forced expression of miR-340 promoted cell viability, proliferation, colony formation and cell cycle progression in the SGC-7901 cells, while miR-340 silencing reduced cell viability, proliferation, colony formation and cell cycle progression in MGC-803 cells. Furthermore, in vivo experiments indicated that miR-340 knockdown suppressed the tumor growth of MGC-803 cells. Notably, alteration of miR-340 expression affected the luciferase activity of wild-type 3'-UTR of cyclin G2 (CCNG2) and regulated CCNG2 abundance in gastric cancer cells, indicating that CCNG2 is a direct target of miR-340. Moreover, CCNG2 knockdown eradicated the effects of miR-340 silencing on gastric cancer cells. In conclusion, our data suggest that miR-340 may potentially serve as a novel prognostic biomarker and therapeutic target for gastric cancer.