The synthesis of iron oxide nanoparticles coated with graphitic carbon nitride (Fex-NC), and their improved electrochemical stability and corrosion resistance in an acidic electrolyte environment are reported. Our results show that the Fex-NC nanocomposites exhibit enhanced activity and long-term stability for the HER in a 0.5 M H2SO4 aqueous solution, with an onset potential of 73 mV and Tafel slope of 69 mV dec−1. Furthermore, DFT calculations are carried out to represent our experimental system. Both theory and experiment strongly correlate with each other, where gC3N4@FeO has superior performance to the pristine gC3N4. It is found that the electrocatalytic activity of gC3N4@FeO arises from the electron transfer from FeO particles to the gC3N4, which form an electrostatic interaction, leading to a decreased local work function on the surface of gC3N4. The resulting graphitic carbon nitride shells prevented direct contact between the iron oxide nanoparticles and acidic electrolyte (H2SO4), so that improved stability and corrosion resistance could be achieved. This work sheds light on new efficient and durable electrocatalysts for applications in acidic environments.