The design of ultrafine, highly efficient and recyclable heterogeneous bimetallic nanoparticles catalysts is challenging. In this work, we report for the first time the preparation of ultrafine and monodispersed bimetallic RhAg nanoparticles that are uniformly supported on reduced graphene oxide (rGO) nanosheets (RhAg/rGO). A key is the presence of the tris(triazolyl)-polyethylene glycol (tristrz-PEG) ligand as a weak stabilizing agent. This amphiphilic tridentate ligand not only enables the formation of ultrafine RhAg NPs, but also allows quantitative fixation of the NPs onto the rGO, which avoids metal loss and further improves catalytic efficiency. The RhAg/rGO catalysts were characterized by various techniques including UV–Vis, ICP-AES, TEM, HRTEM, STEM, EDX and XPS. By varying the molar ratios of Rh to Ag, the highest catalytic activity in the reduction of 4-nitrophenol by NaBH4 was obtained for RhAg0.5/rGO with a remarkable reaction rate of k app = 14.8 × 10−3 s−1 (k nor = 1415 s−1 g−1). Moreover, the catalyst was recycled, and its amount was reduced to 100 ppm of RhAg0.5/rGO while retaining an exceptional catalytic efficiency. The present work contributes to the effective design of ultrafine bimetallic NPs/graphene-based nanocomposites and to the fabrication of very efficient and cost-effective catalysts.