The stress-strain relation for austenitic stainless steels is based on 2 main contributions: work hardening and a phase transformation from austenite to martensite. The transformation is highly temperature dependent. In most models for phase transformation from austenite to martensite, the stress triaxiality plays an important role also. The sensitivity to triaxiality is often investigated based on uniaxial compression and tensile tests. To validate the common formulation for triaxiality dependence of the martensitic transformation, a series of experiments is performed with the Twente biaxial tester for sheet material. A number of deformation directions are prescribed between plane strain and simple shear. Uniaxial tensile tests were performed at different temperatures to get a temperature corrected reference curve for the martensite–strain relation. The current results for typical stress states in sheet forming do not show the dependency on the triaxiality that is given in literature. This means that for sheet forming simulations, changes in stress state affects the martensitic transformation less than expected from tension–compression experiments.