<p>This paper presents the results of an experimental investigation on the shear strength of mismatched rock discontinuities. Direct shear tests were conducted on mortar replicas of three surfaces of different roughness and with increasing degrees of localised mismatch, as opposed to lateral dislocation, introduced both in areas of steep asperities (high gradients) and shallow asperities (low gradients). The peak shear strength was found to reduce with an increasing degree of mismatch in areas of high gradients and stabilises to an approximately constant value once a certain amount of mismatch, referred to as “maximum effective mismatch”, has been reached. The rougher the surface, the more pronounced the effect of mismatch. Removing even a small area of high gradients can halve the peak shear strength. In contrast, the effect of mismatch is marginal in areas of low gradients and almost inexistent on the residual shear strength. It was also found that the loss of strength due to mismatch is roughness dependent but not stress dependent. Two existing models predicting the shear strength of mismatched discontinuities were tested and found to perform poorly. Consequently, a new model was proposed, and much better predictions obtained.</p>