Resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis has been reported in some cancer cells, including AGS human gastric adenocarcinoma cells. Hizikia fusiforme is a commonly used brown seaweed species in Korea that possesses potent antibacterial, antifungal, and anti-inflammatory activities. In this study, we demonstrated that treatment with TRAIL in combination with subtoxic concentrations of ethyl alcohol extract of H. fusiforme (EAHF) sensitized TRAIL-resistant AGS cells to TRAIL-mediated apoptosis. Combined treatment with EAHF and TRAIL increased chromatin condensation, DNA fragmentation, and sub-G1-phase DNA content. The restored sensitivity to TRAIL-induced apoptosis appeared to be correlated with the modulation of Bcl-2 family proteins and activation of caspases, which resulted in the cleavage of poly(ADP-ribose)polymerase. Taken together, the use of EAHF in combination with TRAIL may be an effective and selective anticancer strategy via suppressing the resistance to TRAIL-induced apoptosis in some tumor cell lines, including AGS cells.