In this paper, nitrogen/phosphorous co-doped porous carbons (N/P-PCs) with hierarchical porosity were prepared by a one-step carbonization method, of which nitrogen-containing PAA/MMF is the precursor and phosphorous-containing sodium hypophosphite monohydrate is the novel pore-forming agent. The synergistic effect of multilevel porosity and heteroatomic functionalities results in good electrochemical performance. Tested as electrode materials for supercapacitors, N/P-PC-50 demonstrates a maximal specific capacitance of 177 F g⁻¹ at a current density of 0.1 A g⁻¹, excellent rate capability (67% capacitance retention at 10 A g⁻¹) and superior cycling stability (no capacitance loss after 10 000 cycles). In addition, N/P-PCs have the potential to be used for other applications, and this facile, low-cost and safe synthesis strategy provides a feasible route to develop many hierarchically porous carbon-based materials.