Gastric cancer is the second most common cause of cancer-related deaths in the world. In this study, a bioactive derivative of chitooligosaccharide, named gallic acid-grafted-chitooligosaccharide (G-COS), was evaluated for its capabilities against the proliferation of AGS human gastric cancer cells. It was found that G-COS treatment caused significant inhibition of gastric cancer cell growth at concentrations of 200 and 400 μg mL⁻¹. The anti-growth effect of G-COS in AGS cells was characterized using flow cytometry, fluorescence microscopy, DNA fragmentation and evaluation of protein expression. Notably, G-COS-induced apoptosis was related to the increase in the expression of p53, p21, Bax, and caspases (-9 and -3) and the decrease in the activation of Bcl-2, p-IκB-α and NF-κB (p50 and p65). These findings indicate that G-COS has potential to be applied in the treatment of gastric cancer as a cancer chemopreventative agent.