This article presents the surface activity of a synthetic sulfonated phenolic resin (SPR) and evaluates its performance in cement mortars. It is indicated that the prepared SPR has the characteristics of a polymeric surfactant that can decrease the surface tension of water. When resin was added into cement mortars, the resulting materials exhibited better workability, more consolidated structure, and higher compressive strength than the untreated. Therefore, SPR could be used as a water reducing admixture for cementitious materials. In addition, resins with higher sulfonate group content and low molecular weight appear to be more effective.