To provide a salt-resistant water-retaining agent which can keep stable water-absorption performance even under a high ion concentration, and a greening material using the water-retaining agent.

The salt-resistant water-retaining agent comprises a crosslinked copolymer comprising N-alkoxymethyl (meth)acrylamide (A) and a crosslinkable monomer (B) having two or more vinyl groups as the monomer units, wherein the copolymer further comprises one or more kinds of ethylenically unsaturated monomer units (C) as the monomer units copolymerizable with components (A) and (B). The greening material comprises the water-retaining agent.