In this study, we investigated the effects of 2,3,6-tribromo-4,5-dihydroxybenzyl methyl ether (TDB), isolated from Symphyocladia latiuscula (marine red algae), on the proliferation of MCF-7 human breast cancer cells. TDB treatment for 48 h inhibited cancer cell growth and induced DNA fragmentation. Furthermore, morphological characterizations such as apoptotic bodies and membrane blebs were shown by electronic microscopy. TDB-induced apoptosis in the MCF-7 cells was closely linked with the down-regulation of Bcl-2 protein expression and the cleavage of caspase-3 substrates, with poly(ADP-ribose) polymerase cleavage occurring by TDB treatment. TDB treatment also caused a marked increase in the level of $p21^{WAF1/CIP1}$ protein in a p53-dependent manner. In addition, the upregulation of$p21^{WAF1/CIP1}$ in the MCF-7 cells was related to a decrease in c-Myc protein in a dose-dependent manner. Based on our data, TDB is a good candidate for further evaluation as an effective chemotherapeutic agent, acting through the induction of apoptosis.