Herein, we developed a facile one-step strategy to construct uniform bimetallic Pt75Co25 alloyed nanodendritic assemblies (Pt75Co25 NDAs) in oleylamine (OAm). OAm and cetyltrimethylammonium chloride (CTAC) acted as reductant and structure-director, respectively. l-Proline served as the green co-reductant and co-structure-directing agent. By virtue of the unique structures and cooperative effects of the bimetals, the obtained catalyst displayed remarkable enlargement in catalytic properties for hydrogen evolution reaction (HER) in 0.5 M H2SO4 at 1600 rpm with a scan rate of 5 mV s−1 and oxygen reduction reaction (ORR) in 0.5 M KOH at 5 mV s−1 with a rotation rate of 1600 rpm, outperforming home-made Pt60Co40 nanoparticles (NPs), Pt82Co18 NPs and commercial Pt black catalysts. This work offers some guidance for novel dendritic nanoassemblies as advanced efficient and robust electrocatalysts. Display omitted •Pt75Co25 NDAs were fabricated by a one-pot method.•l-Proline and CTAC acted as the co-structure-directors.•The specific architectures have highly enlarged specific surface area.•The catalyst showed dramatically boosted catalytic performances for HER and ORR.