Seaweeds are commonly used as functional foods and drugs. A glycoprotein (GP) from the green alga Capsosiphon fulvescens (Cf) has been reported to have antitumor activity toward various cancer cells. We previously observed that Cf-GP induced different pathways of apoptosis in AGS human gastric cancer cells. Transforming growth factor (TGF)-β1 plays an important role in cancer cell migration. Increased TGF-β1 levels increase the expression of the small GTPases and activate the FAK/PI3K/AKT pathways, resulting in the upregulation of integrin receptor proteins, which mediate the attachment of cells to surrounding tissues, cells or extracellular matrix. Thus, the inhibition of TGF-β1 signaling would downregulate integrin expression and thereby effectively decrease cell growth and migration. In the present study, we determined the effect of Cf-GP treatment on the proliferation, migration and apoptosis of AGS human gastric cancer cells. To investigate the mechanism by which Cf-GP exerts its anticancer actions, we examined the effect of Cf-GP on the expression levels of TGF-β1, FAK, PI3K, AKT, the small GTPases and integrins in AGS cells. Our findings indicate that Cf-GP inhibits AGS cell proliferation and migration by downregulating integrin expression via the TGF-β1-activated FAK/PI3K/AKT pathways. These results suggest that Cf-GP may be an important factor in the development of functional foods and therapeutic agents.