Superplasticizers are crucial materials for cementitious materials to enhance workability while minimizing water usage and increasing the product strength. However, there is a challenge to apply superplasticizers in rapid processes, such as the gypsum board fabrication process, that require good plasticization properties and fast hydration. For the first time, we report a novel sulfonated polymer nanoparticle (S-NP) as a superplasticizer suitable for gypsum board. The S-NP was synthesized using styrene, sodium 4-styrenesulfonate, and divinylbenzene through eco-friendly surfactant-free emulsion polymerization. The dispersion/plasticization effect of S-NP was investigated in terms of fluidity, water reduction property, and mechanical properties. The S-NP with its specific coiled conformation exhibited superior fluidity and water reduction properties compared with commercially available polynaphthalene sulfonate (PNS) due to improved polymer adsorption on the hemihydrate particles. The S-NP exhibited similar hydration kinetics to the pure hemihydrate and provided higher mechanical strength than PNS due to smaller pore size and denser microstructure in the gypsum product. These findings proved that S-NP is a more suitable alternative superplasticizer for the gypsum industry.