In this work, the effect of particle size and the content of leather wastes on the processability, vulcanization, and mechanical properties of a natural rubber/leather wastes composites was studied. From a reduction process, leather waste was later sieved to classify particles smaller than 0.6 mm and particles between 0.84 and 2 mm, hereafter called fine and coarse residues, respectively. The morphology of the wastes was analyzed from scanning electron microscopy micrographs. Natural rubber/leather wastes composites with 20, 40, 60, 80, and 100 phr (parts per 100 parts of rubber) of treated leather wastes were obtained in a torque rheometer. Mixtures with coarse wastes exhibited a slightly higher torque than mixtures with the fine wastes; additionally, increasing the wastes content yields in an increased torque during the final stage of the process. The addition of leather wastes increased the vulcanization time and stiffness of the composites; additionally, materials with fine wastes showed lower vulcanization times compared to materials with coarse wastes. Composites with fine leather wastes exhibited higher density and tensile modulus than materials with coarse wastes; however, only the composites with 80 and 100 phr of wastes showed a statistically significant difference in hardness values. The specific energy consumption (SEC) was calculated from the results obtained with the torque rheometer; it was found that for all the formulations the SEC decreased as the phr of leather residues increased. It was found that it is possible to use leather wastes in natural rubber composites, to obtain adequate materials feasible for some applications.