A highly absorbent resin which is essentially a polyamine-crosslinked, partial neutralization product obtained by reaction of an alkali metal hydroxide with a carboxyl-containing polymer selected from the froup consisting of alpha-olefin-maleic anhydride copolymers, vinyl compound-maleic anhydride copolymers, polyacrylic acid, polymethacrylic acid, and mixtures thereof, the degree of neutralization of the said neutralization product being 0.4 to 0.8 equivalent of total carboxyl groups of the said carboxyl-containing polymer, the amount of polyamine as a crosslinking agent being not more than 2 parts by weight per 100 parts by weight of the neutralization product on the uncrosslinked basis, the resin being capable of absorbing at least 20 times its own weight of distilled water. The resin has good resistance to heat and good durability in the hydrous condition.