A fucoidanase from the hepatopancreas of Patinopecten yessoensis was purified by ammonium sulfate precipitation, anion exchange chromatography, isoelectric focusing, and gel chromatography. The purified enzyme gave a single band on polyacrylamide gel electrophoresis. The fucoidanase was practically free from α-L-fucosidase and arylsulfatase activities. The molecular weight of the enzyme was estimated to be 85,000 by gel filtration on TSKgel G3000SW and 84,000 by SDS (sodium dodecyl sulfate) polyacrylamide gel electrophoresis. The enzyme hydrolyzed fucoidan to produce sulfated oligosaccharides as the reaction products.