The realization of small platinum (Pt) nanoparticles (NPs) embedded in conductive porous carbon would largely improve the catalytic activity effectively with more durability, although there remain challenges to achieve such hybrid nanostructures via a simple synthetic route. Here, an efficient and facile profile was demonstrated for the synthesis of one kind of uniform three-dimensional (3D) spherical Pt/C composite electrocatalyst with small monodispersed Pt NPs embedded in the matrix of 3D spherical porous carbon derived from the corresponding spherically polymeric Pt(II) complex. The monodispersed Pt NPs within the uniform 3D Pt/C composite are ∼4.4 nm and they are dispersed homogeneously within the matrix of 3D spherical porous carbon. Investigations showed that the 3D Pt/C composite exhibits high catalytic performances as compared to the commercial catalyst of Pt black for oxidation reactions of ethylene glycol, ethanol, and methanol. This strategy developed in the present study would be available for possible fabrication of some other active 3D porous carbon-supported Pt-based catalysts including their bimetallic and multimetallic counterparts.