Objective(s): This paper aims to investigate synergistic inhibition of polysaccharide from Sepia esculenta ink (SIP), a newly isolated marine polysaccharide in our laboratory, on breast cancer MDA-MB-231 cells exposed to cisplatin. Materials and Methods: Cell viability of MDA-MB-231 cells was determined by CCK 8 assay. Median-effect concentration was analyzed using Chou-Talalay method that was also subjected to determine cell inhibition ratio and combined index, as well as interaction between SIP and cisplatin. Proliferation and migration abilities were detected with plate colony formation assay and cell wound scratch assay, respectively. Expression of MMP-2 and MMP-9 proteins was measured with Western blot assay. Results: Data showed that SIP not only suppressed proliferation and migration of MDA-MB-231 cells, and expression of MMP-2 and MMP-9 proteins, also promoted inhibition of cisplatin on proliferation, migration and MMPs expression of MDA-MB-231 cells, which indicates synergy inhibition of drug combination of SIP and cisplatin on breast cancer cells. The median-effect concentrations of cisplatin and SIP were 4.9 and 1659.6 μg/ml, respectively. Whereas the concentration of combination drug was 158.5 μg/ml. The data indicated that drug combination can decrease dosages of the two single agents, especially the usual dosage of cisplatin. Conclusion: This research demonstrated that SIP repressed proliferation and metastasis of MDA-MB-231 cells and promoted anticancer effect of cisplatin on the breast cancer cells. The data suggested that SIP is a potential natural drug that can be used as an auxiliary medicine alongside chemotherapy in treating breast cancer.