We developed a solvothermal process by a reflux method for growing graphitic carbon nitride (g-C3N4) polymer on graphene oxide (GO) to form three-dimensional (3D) flake-like g-C3N4 on a reduced GO (g-C3N4@RGO) hybrid support material. The as-synthesized g-C3N4@RGO, depending on the synthesis ratio of GO to melamine, showed excellent specific capacitance (220–258 F g−1) at 10 mV s−1, a very low charge transfer resistance, high N contents (6–18 at.%), and a 3D network structure. Additionally, compared with Pt-loaded reduced GO (Pt/RGO), when Pt was loaded on g-C3N4@RGO for catalysis, Pt nanoparticles (NPs) with diameters of 2–4 nm were found to be uniformly dispersed. In direct-methanol fuel cells (DMFCs), the Pt/g-C3N4@RGO (8:1) nanocatalysts exhibited high utility in terms of their electrochemically active surface area (ECSA) (81.5 m2 g−1), electrocatalytic oxidation current density of methanol (13.7 mA cm−2), CO poison tolerance (IF/IR = 1.38), and duration stability, compared with Pt/RGO, Pt-loaded Vulcan XC-72 (Pt/C), and Pt-loaded graphite (Pt/Gr). These results demonstrate the potential of as-synthesized g-C3N4@RGO hybrid material as a high-performance catalyst support for fuel-cell applications.