This paper investigates the behavior of EPS geofoam in a true triaxial apparatus using 70mm×70mm×140mm prismatic brick-shaped specimens. The specimens are subjected to different stress paths in the deviator (π) plane by means of stress-controlled loading, in which the axial stress is imposed at a rate of 75kPa/min in the major principal direction. Stress–strain characteristics and volume change behavior have been recorded, and the yield surface has been deduced from the experimental data. The following observations have also been made for the geofoam: (a) it is an elastoplastic hardening material with plastic contractive volume change under compressive loading, (b) it softens stiffness-wise under confining stress, (c) the onset of contractive volume change corresponds quite well to the proportional limit, and (d) yielding is a slightly decreasing function of the intermediate principal stress. The study found that yielding can be represented reasonably well by a Drucker–Prager yield surface.