The effects of the stress state and temperature on the martensitic phase transformation behavior in a TRIP-assisted steel (TRIP780) were investigated using multi-axial experimental techniques. For this purpose, five different stress states were considered; i.e., uniaxial tension, uniaxial compression, equi-biaxial tension, plane strain tension and simple shear. A range of temperatures from room to 100°C for each stress state condition except the simple shear test were investigated. In particular, for the equi-biaxial tension data in warm conditions, a specially designed hydraulic bulge experiment was adopted. In situ magnetic measurements were performed to monitor the evolution of the martensitic content throughout each experiment. A stress state and temperature dependent transformation kinetics law was proposed, which incorporates a non-linear function of the stress triaxiality, Lode angle parameter and temperature. This new model captures the measured martensitic phase transformation kinetics of TRIP780 steel over a wide range of stress states and temperature reasonably well.