Fucoidan exhibits various biological properties. We raised a novel antibody against fucoidan extracted from Cladosiphon okamuranus and developed a sandwich ELISA method to measure fucoidan. The fucoidan antibody was specific and did not cross-react with other polysulfated polysaccharides. Fucoidan recovery from serum and urine by ELISA was 86-113%. Intra- and inter-assay CVs were 1.5-13.4%. Assay linearity was maintained after 3-fold dilution of each sample with phosphate-buffer saline (PBS). In the serum and urine of healthy volunteers (n=10), fucoidan was not detected before administration, and the levels markedly increased 6 and 9 h after oral administration. The molecular weight of the serum fucoidan determined by HPLC gel filtration remained unchanged, whereas that of urine fucoidan was significantly reduced. This is the first ELISA method of measuring serum and urine fucoidan levels after oral administration. The method is simple, reliable, and practical for the analysis of samples, especially urine samples.