The aim of this investigation is to study the effect of stress state on the kinetics of the retained austenite to martensite transformation in medium manganese TRIP steels. Tensile experiments were performed on two different sample geometries, in addition to tests on classical dogbone samples, to assess stress states with varying stress triaxialities. Low and high stress triaxialities were achieved through the optimization of two different sample geometries. For this, a finite element-based optimization cycle was implemented in order to determine the critical dimensions to achieve a stress state with constant triaxiality in a cube with a 1 mm side in the center of the specimen. The optimized in-plane shear, uniaxial, and plane strain samples generated constant triaxialities of ~0, ~0.3, and ~0.6, respectively, up to 10% plastic equivalent strain. The samples were tested at quasi-static strain rates using a conventional tensile device. Digital image correlation was used to obtain the strain fields in the samples throughout the deformation process. The evolution of transformation from retained austenite to martensite was measured via XRD, interrupting the tests at different strain levels. The analyses focus on the plastic deformation region from the onset of yielding up to localization of the strain.