A rational strategy is developed to prepare a nanocomposite with OD/3D architecture. The composite containing Mn3O4 nanoparticles (OD) and carbon foam (3D) could be applied as an electrode material for a supercapacitor by taking advantage of the high conductivity of carbon foam (CF) and high pseudocapacitance of Mn3O4 nanoparticles. CF is prepared by a carbonization method using melamine foam, and then Mn3O4 nanoparticles are combined with carbon foam by a one-step hydrothermal method to prepare the Mn3O4@CF nanocomposite. The OD@3D hierarchical structure of Mn3O4@CF nanocomposite using CF as a 3D growing skeleton prevents agglomeration and increases reactive sites of Mn3O4 nanoparticles. In addition, CF as a conductive skeleton shortens the charge transfer path. The synergistic effect between CF and Mn3O4 improves the electrochemical performance of CF. Three Mn3O4@CF composites are prepared by adjusting the mass of the reactants in the processes of hydrothermal reaction. The Mn3O4 nanoparticles are uniformly grown on the CF surface with a diameter of 10 nm. Mn3O4@CF-2 composite has a specific capacitance of 212.8 F/g at a current density of 1 A/g, which is much higher than that of pristine CF (79.1 F/g) and Mn3O4 (112.7 F/g). The cyclic stability of Mn3O4@CF-2 is retained as 86.1% of initial capacitance after 2000 cycles at the current density of 1 A/g. It proves the feasibility of the as-mentioned strategy and broadens the application of carbon foam in supercapacitors.