Morphological parameters of the mammalian kidney can provide information on the extent to which an animal is able to regulate water relations within its body and cope with arid conditions. Certain groups of mammals, such as rabbits and hares, show a flexible phenotype (phenotypic plasticity) with respect to renal physiology that enables them to inhabit a variety of bioclimatic regions. We investigated kidney morphology in the European rabbit, Oryctolagus cuniculus, located on two isolated islands off the coast of the Western Cape South Africa, at sites with varying water and food availability, to ascertain whether these populations demonstrate differences in renal morphology that would link to differential renal physiology. In summer, Dassen Island is more restricted in available water and plants than Robben Island; we tested the prediction that rabbits from Dassen Island would be physiologically more suited to water-limited conditions. Our data showed that O. cuniculus exhibits phenotypic plasticity, with Dassen Island rabbits displaying renal adaptations more suited to survival in drier environments, such as an increased relative medullary thickness and a higher predicted maximal urine-concentrating ability. This suggests increased renal performance in the Dassen Island population with the consequent increased ability to concentrate urine and conserve body water.