A water retention agent for a cementitious composition, characterized in that it takes the form of a liquid aqueous suspension of at least one polysaccharide at a mass concentration of between 15 and 30% in an aqueous solution of a strong base salt, excluding ammonium salts, with an anionic strength of between 1.25 mol/L and 15 mol/L, having a pH greater than 9 and containing an attapulgite in micronized form and at least one non-phyllitic mineral powder, referred to hereafter as filler, which is chemically inert in the aqueous suspension and which has a grain size of between 0.1 and 100 micrometers, the aqueous suspension being stable at least in a temperature range of between 5° C. and 30° C. The water retention agent is suitable for increasing both the viscosity and the water retention capacity of cementitious compositions without affecting the spreading ability thereof.