Nitrogen-doped ordered mesoporous carbon (NOMC) with a cubic Imm symmetry and rhombic dodecahedral single-crystal morphology has been successfully synthesized for the first time via organic–organic self-assembly of triblock copolymer, 3-aminophenol, and hexamethylenetetramine (HMTA) in basic aqueous solution. A steam treatment at elevated temperature has been developed to remove the surfactant from the as-synthesized sample, open the mesoporous cages, and create abundant micropores in the final product. Benefiting from the unique features of high surface area, uniform and uninterrupted mesopores, rich microporosity, and moderate nitrogen-doping (3.42%), the resultant NOMC single crystals show a high capacitance (281 F g⁻¹ at 0.5 A g⁻¹), excellent rate capability (195.5 F g⁻¹ at 20 A g⁻¹), and outstanding cycling stability (97% capacity retention after 5000 cycles at 5 A g⁻¹) in electrochemical double-layer capacitors (EDLCs).