Gastric cancer (GC) is the most common cause of morbidity and mortality because of cancer. Medicinal plants containing polyphenolic compounds have gained importance in anticancer treatment. In this context, carvacrol is a main component of many plants in the family Lamiaceae that are frequently used in folk medicine and a good candidate to investigate for GC treatment. The present study aimed to explore the cytotoxic, genotoxic, apoptotic, and reactive oxygen species (ROS)-generating effects of carvacrol on gastric adenocarcinoma in vitro. For these purposes, human gastric adenocarcinoma (AGS) cells were used and analyzed after 24 h of exposure to carvacrol with different concentrations. The cytotoxicity, ROS generation, glutathione (GSH) level, and genotoxicity were investigated by the ATP cell viability assay, 2′,7′-dichlorodihydrofluorescein-diacetate assay, GSH/GSSG-Glo assay, and comet assay, respectively. Apoptosis induction was detected by acridine orange/ethidium bromide staining and western blotting at below the half-maximal growth inhibitory concentration value. Carvacrol showed cytotoxic, genotoxic, apoptotic, ROS generating, and GSH-reducing effects on AGS cells in a dose-dependent manner. There was a close negative relationship between cell viability and ROS level. Carvacrol inhibited the proliferation of AGS cells, suggesting that it could be a novel and strong anticancer agent against the human gastric adenocarcinoma. These results support the interest of natural diet components in the development of therapeutic products for diseases.