The medium-Mn steel is a promising third-generation automobile steel. Its chemical composition, microstructure, and thermal and mechanical properties are introduced and a warm-stamping process for the medium-Mn steel is proposed. The optimal process parameters are identified through the design of experiments (DOE) and range analysis. The evaluated experimental indexes include tensile strength, elongation, and hardness. The optimal forming process consists of an austenitization temperature of 840 °C, a soaking time of 4 min, and an initial stamping temperature of 500 °C. The proposed process was applied to the warm stamping of an automotive B-pillar. The microstructure of ultrafine, uniform, and complete martensite laths was obtained. The formed part exhibits approximately 1420 MPa tensile strength, over 11% elongation and 460 HV hardness. The optimal warm-stamping process has proved effective and applicable for forming medium-Mn steel parts. It will help promote the application of the third-generation automotive steels.