A cement dispersant which displays high dispersibility with a small addition amount and excellent dispersibility particularly even in a high water-reducing ratio area, and a cement composition comprising this. A first cement dispersant includes a copolymer having a first constitutional unit derived from an unsaturated (poly)alkylene glycol ether monomer; and a second constitutional unit derived from an unsaturated monocarboxylic acid monomer, wherein the second constitutional unit includes at least one structure derived from an acrylic acid (salt).