Oleuropein (OLE), the main phenolic compound of the olive fruit and leaves, has many heathful effects. Gastric cancer is the most fatal malignancy in many parts of the world and it is generally related to harmful dietetic factors. The anticarcinogenic role of OLE in gastric cancer has not been studied sufficiently yet. In this study, we aimed to research the cytotoxic, genotoxic and apoptotic effects of OLE on gastric adenocancer (AGS) cells in vitro. A standard cell line derived from gastric adeno cancer (AGS) cells was employed, and its performance following a 24-hour exposure to OLE at various doses was examined. The ATP cell viability assay, 2',7'-dichlorodihydrofluorescein-diacetate assay (H2DCF-DA) and alkaline single cell gel electrophoresis assay (Comet Assay) were used to study the cytotoxicity, production of reactive oxygen species (ROS) and genotoxicity respectively. The induction of apoptosis was discovered using flow cytometry. OLE reduced AGS cells viability about 60% at maximum concentration (500 µmol/L) and also resulted in approximately 100% DNA damage and about 40% apoptosis with necrosis in AGS cells depending on the increased doses. Cell viability was also significantly decreased in relation to increased intracellular reactive oxygen species (ROS) levels (p < 0.05 - 0.001). Oleuropein has shown significant anticarcinogen effects against gastric adenocancer (AGS) cells in vitro. Oleuropein, a nutrient rich in olive and olive oil, seems to be both protective and therapeutic against gastric cancer and may be a new chemotherapeutic agent in the future.