A whey protein with a phosphorus content reduced to 0.15 mg or less per gram of the protein, a process for the production thereof, a hydrolyzate of a highly purified low-phosphorus whey protein, and a process for the production thereof. The low-phosphorus whey protein is produced by adjusting the pH of a whey protein solution to 4 or below and bringing the solution into contact with a cation-exchange resin and an anion-exchange resin successively to thereby reduce the phosphorus content to 0.15 mg or less per gram of the protein. The hydrolyzate of the highly pure low-phosphorus whey protein is obtained by hydrolyzing the low-phosphorus whey protein with protease.