This paper presents results of a laboratory investigation of the influence of the nonuniform stresses imposed by the Geonor direct simple‐shear (DSS) apparatus on measured stress‐strain behavior. The investigation concentrated on the influence of the apparatus on the measured strain‐softening behavior of cohesive soils during constant volume (undrained) shear. Special DSS tests were conducted on an elastic material and cohesive soils at varying overconsolidation ratios. The experimental results clearly indicate that the non-uniformity of stresses imposed by the device during undrained shear causes a decrease in the vertical effective stress with increasing shear strain. This in turn causes a reduction in the measured horizontal shear resistance at large strains. The device‐induced “unloading” probably has relatively little effect on measured peak strengths in K0 consolidated‐undrained DSS tests but certainly contributes to the pronounced strain softening typically observed.