Background: Garlic, a common spice used since time immemorial for various purposes, is considered a potential functional food as it exhibits cardioprotection to chemoprevention properties due to the presence of organosulfur constituents in each garlic pod. Alliin is not widely studied for its bioactivity as it is an unstable compound which is converted to allicin on mechanical and chemical degradation. Objective: Hence, in the present study, the influence of alliin on gastric adenocarcinoma (AGS) cells and normal intestinal cells (INT-407) would be tested for its potent antiproliferative effect and mechanism of action. Materials and Methods: The quantity of alliin in fresh and dried garlic extract was measured by high-performance liquid chromatography to corroborate the cytotoxicity activity identified by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and acridine orange/ethidium bromide staining. Further, to identify the mechanism of action, DNA fragmentation assay, annexin V Assay, and flow cytometry analysis of apoptosis followed by expression of apoptotic proteins such as Bax, Bcl-2, and cytochrome-C by Western blot was done. Results: It was identified that alliin inhibited proliferation of gastric carcinoma cells by decreasing the cell viability but not in the normal intestinal cells. The level of apoptosis was modulated by reactive oxygen species generation and decrease in mitochondrial membrane potential mediated by deregulation of Bax/Bcl-2 level at protein level leading to upregulation of Cytochrome C. Conclusion: It is impressive to note that alliin content was high in fresh aqueous extract compared to that of dried garlic extract which concludes that the use of garlic from time immemorial is a worthy functional food in its fresh form to combat cancer cells. However, in-vivo studies are warranted.