This paper describes a facile and surfactant-free synthesis of a SnO₂ modified truncated octahedral Pt₃Ni alloy nanoparticle electrocatalyst by using neat N-formylpiperidine as the solvent, which is capable of controlling the formation of the Pt₃Ni–SnO₂–C triple junction nanostructure, and the shape/surface composition of Pt₃Ni nanoparticles. The area-specific oxygen reduction reaction (ORR) activity for the Pt₃Ni/SnO₂–C catalyst reaches up to 2.67 mA cm⁻², which is 2.4 times that of the Pt₃Ni/C catalyst. The electrochemical stability of the Pt₃Ni/SnO₂–C catalyst is 2.5 times as high as that of Pt₃Ni/C catalysts. The significant improvement in ORR activity and durability can be mainly ascribed to the strong interaction of the Pt₃Ni alloy and SnO₂ and the anchoring effect of Pt₃Ni nanoparticles by both SnO₂ and C at the Pt₃Ni–SnO₂–C triple junction nanostructure as well as the Pt-rich surface composition, which could hinder the dissolution of cheap Ni metal. The improved catalytic behavior in the ORR highlights the importance of surfactant-free synthesis and the unique shaped metal alloy–metal oxide–carbon triple junction nanostructure.