The leather industry, particularly shoe manufacturing, generates leather scraps mainly dumped in domestic or industrial waste landfills. Leather waste from upper material cutting operations represents 50-70% of solid waste produced by footwear companies, resulting in around 150 000 tons of this type of waste produced annually in Europe. Vulcanised rubber is widely applied in medium to high-quality shoe soles. Leather waste fibres added to elastomers can function as short fibre reinforcement for the matrix as long as their inherent fibrous nature is maintained during processing. In this study, we report the preparation of SBR reinforced with leather waste from the shoe industry. We evaluate the influence of leather fibre size presence on composites' properties. In addition, the effect of different leather fibre concentrations on composite properties was also accessed. The composites were produced by vulcanisation, preceded by mixing with either a laboratory-scale open two-roll rubber mixing mill or an internal mixer machine – two practical processing techniques, versatile and compatible with large-scale industrial processes. Composite properties were assessed by rheology, FTIR and thermogravimetry and the mechanical properties were accessed by tensile tests. In addition, the morphology of composites was observed by a SEM. The results indicated that the leather fibre size affected the leather dispersion in the composites. For example, regarding the mechanical properties, despite a tendency to reduce tensile strength with increasing concentration of leather fibres, it was found that CompA (processed with a leather fibre size of 400-600 µm) exhibited a slight improvement of around 14% in tensile strength when compared with pure SBR, suggesting the potential of leather waste to be used with SBR to produce composites with better mechanical properties. On the opposite side, CompB (processed with a leather fibre size of 1-2 mm) displayed a tensile strength of around 30% less than CompA, indicating that the leather fibre size plays a fundamental role in the final performance of the composites, related to the ability of the shorter fibres to disperse better in the SBR matrix, also leading to less voids at the fibre-SBR interface.