Gypsum is widely used for the manufacture of various building materials because of its advantages such as rapid setting, good thermal insulating property and fire resistance, but their use is limited to interior finishings because of its poor water resistance. This paper deals with an improvement in the water resistance of gypsum composites by polymer modification and partial replacement of gypsum with ground granulated blast-furnace slag, silica fume and ordinary Portland cement. Polymer-modified gypsum-based composites are prepared with various polymer-binder (a mixture of gypsum and mineral admixtures) ratios, and tested for water absorption, flexural and compressive strengths before and after water immersion. In addition, the long-term water resistance of the polymer-modified gypsum-based composites with a superplasticizer are investigated by 3-y water immersion. As a result, SBR modified gypsum-based composites with optimum mix proportions are developed from the viewpoint of water resistance and strength development. The high water resistance of the gypsum-based composites is achieved by polymer modification, superplasticizer addition, and partial replacement of gypsum with the mineral admixtures.