Advanced numerical modelling of cold-formed stainless steel members, from manufacturing to ultimate failure under loading, requires knowledge of the stress-strain relationship of the material over a wide range of strains. Although a number of stress-strain models exist, they are only capable of accurate predictions either over a limited strain range or for the tensile stress-strain behaviour only. This paper presents a three-stage stress-strain model for stainless steels which is capable of accurate predictions over the full range of both tensile and compressive strains. The new stress-strain model is defined by the basic Ramberg-Osgood parameters (E, σ and n) and is based on a careful interpretation of existing experimental data. The accuracy of the proposed model is demonstrated by comparing its predictions with experimental stress-strain curves.