Capsosiphon fulvescens is well-known green sea algae that, in recent years, has been proposed as a potential anticancer drug. In this study, we found that C. fulvescens glycoprotein (Cf-GP) had pro-apoptotic effects on human gastric carcinoma cells. By SDS-PAGE, we confirmed that C. fulvescens extract contained a glycoprotein. Using H33342 staining, we found that the Cf-GP caused cell death in a does-dependent manner, while an MTS assay showed decreased cellular viability due to induction of apoptosis. To determine the effect of Cf-GP on apoptosis-related cellular events, cells were treated with Cf-GP and the expression of several apoptosis-related protein was determined by Western blotting. Our results indicate that Cf-GP activated both a caspase cascade and PARP, which is a substrate of caspase-3, caspase-8 and the Bcl-2 family proteins. In addition, we assessed caspase-3, and -8 activation and annexin V staining. Our results revealed a cell cycle arrest, itself leading to an increased percentage of sub-G1 cells. Our findings indicate that Cf-GP may be a source of bio-functional material with therapeutic effects on human gastrointestinal cancer.