It is shown, using the nonlinear sigma-model technique, that nonmagnetic disorder leads to an increase of the indirect RKKY interaction of magnetic impurities diluted in a disordered metal. In the weak-disorder limit the root-mean-square (r.m.s.) value of the interaction does not depend on the disorder parameter and the main effect of the disorder is in causing significant mesoscopic fluctuations of the interaction. On approaching the Anderson transition in d > 2 dimensions or a region of strong localization in d = 2, all the even moments of the interaction are critically increased so that both the r.m.s. value and the fluctuations become stronger, and the distribution over the values of the interaction proves to be logarithmically normal.