Docosahexaenoic acid ethyl ester (DHA-Et) was purified by adsorption on Ag-ion-immobilized membranes via selective interaction between silver ion and carbon-carbon double bonds of DHA-Et. Silver ions were immobilized onto sulfonic-acid-group-containing porous hollow-fiber membranes at an Ag ion density of 1.4 mol/kg of membrane, and 30 membranes were housed in one module (inner diameter = 18 mm and effective length = 80 mm). The adsorption isotherms of DHA-Et in various organic solvents revealed that DHA-Et was adsorbed on the immobilized Ag ions with a DHA-Et/Ag ion molar binding ratio of 1/5 in methanol, and that acetonitrile was the solvent of choice for the elution of the adsorbed DHA-Et. Permeation of bonito oil ethyl ester solution in methanol through the Ag-ion-immobilized hollow-fiber membrane module demonstrated that the displacement adsorption of other lower unsaturated fatty-acid ethyl esters by DHA-Et proceeded along the membrane thickness. The purity of DHA-Et was improved to 99 wt % by permeating first bonito oil ethyl ester containing 95 wt % DHA-Et and then acetonitrile through the module.