An object of the present invention is to provide: a water-soluble polymer which can exercise a high calcium-ion-binding ability and a high clay dispersibility even under circumstances where high-hardness water is used; and a process for producing such a water-soluble polymer with ease and good productivity. As a means of achieving this object, the water-soluble polymer according to the present invention has: (1) a calcium-ion-binding ability of not less than 470 mgCaCO3/g and further a clay dispersibility of not less than 0.90 in a test liquid having a calcium concentration of 50 ppm in terms of calcium carbonate; or (2) a calcium-ion-binding ability of not less than 430 mgCaCO3/g and further a clay dispersibility of not less than 0.70 in a test liquid having a calcium concentration of 100 ppm in terms of calcium carbonate.