Hyperlipidemia is associated with increased risk of the development of cardiovascular diseases. Although a great deal of attention has been paid to the hypolipidemic activity of fucoidan, complex polysaccharides from brown seaweeds, the underlying mechanism is still unclear. This study was performed to investigate whether and how fucoidan has lipid-lowering potential in poloxamer-407 (P407)-induced hyperlipidemic mice. Fucoidan treatment 2 h after acute administration of P407 in these mice significantly reduced serum total cholesterol, triglycerides, and LDL cholesterol levels, but increased the levels of HDL cholesterol. In HepG2 hepatocytes and the liver, fucoidan decreased the expression of FAS and ACC mRNA with no or only a moderate inhibitory effect on SREBP-1c mRNA expression. Furthermore, fucoidan attenuated the hepatic expression of mature SREBP-2 protein with a subsequent decrease in hepatic HMG-CoA reductase mRNA expression and an increase in hepatic LDL receptor mRNA expression. In addition, atherosclerotic lesions in the aorta of chronically P407-treated mice were also reduced by fucoidan. These findings indicate that fucoidan improves serum lipid levels by regulating the expression of key enzymes of cholesterol and triglyceride syntheses in the liver through modulation of SREBP-2.