Studies of seaweeds are essential as these marine macro algae are the only source that produces agar, alginate and carrageenan. Eucheuma cottonii belongs to the genus Eucheuma, in the division of Rhodophyta. It is one of the important genera of carrageenophytes. It was reported to possess anticancer effects for the prevention and management of cancers. This study investigates the cytotoxic effect of crude extracts from E. cottonii on breast cancer cell line (MCF-7) in vitro. E. cottonii collected from Sabah was used for preparation of crude extracts using absolute methanol. The cytotoxic effect of the extract was determined using MTT assay whereas cell apoptosis was detected using GeneTex Enhanced Apoptotic DNA Ladder Detection Kit. Result of MTT assay shows that the growth of MCF-7 cancer cells was suppressed by the crude extracts of E. cottonii in a dose-dependent manner of IC50 at 3.5mg/mL (24h) and 0.85mg/mL (48h). The presence of fragmented DNA of cells treated with the crude extract indicated cytotoxic effect is through apoptosis. In conclusion, E. cottonii extract that exerts cytotoxic effect on the MCF-7 cancer cell line is dose dependent and via apoptosis induction. It can be a potential source of anticancer drugs from nature.