Marine macroalgae consist of a range of bioactive molecules exhibiting different biological activities, and many of these properties are attributed to sulfated polysaccharides, fucoxanthin, phycobiliproteins, and halogenated compounds. In this study, a glycoprotein (GLP) with a molecular mass of ∼48 kDa was extracted and purified from Codium decorticatum and investigated for its cytotoxic properties against human MDA-MB-231 breast cancer cells. The IC 50 values of GLP against MDA-MB-231 and normal breast HBL-100 cells (control) were 75 ± 0.23 μg/mL (IC25), 55 ± 0.32 μg/mL (IC50), and 30 ± 0.43 μg/mL (IC75) and 90 ± 0.57 μg/mL (IC25), 80 ± 0.48 μg/mL (IC 50), and 60 ± 0.26 μg/mL (IC75), respectively. Chromatin condensation and poly(ADP-ribose) polymerase (PARP) cleavage studies showed that the GLP inhibited cell viability by inducing apoptosis in MDA-MB-231 cells. Induction of mitochondria-mediated intrinsic apoptotic pathway by GLP was evidenced by the events of loss of mitochondrial membrane potential (ΔTSIm), bax/bcl-2 dysregulation, cytochrome c release, and activation of caspases 3 and 9. Apoptosis-associated factors such as reactive oxygen species (ROS) formation and loss of ΔTSIm were evaluated by DCFH-DA staining and flow cytometry, respectively. Cell cycle arrest of G2/M phase and expression of apoptosis associated proteins were determined using flow cytometry and Western blotting, respectively. © 2014 American Chemical Society.