Abstract: Because of its high concentration of in-plane elemental nitrogen, superior chemical/thermal stability, tunable electronic band structure and environmentally friendly nature, graphite-like carbon nitride (g-C₃N₄) is a new promising metal-free material that has drawn much attention in photo-/electric catalysis. Compared with the regulation of the band structure in photocatalysis, the deliberate synthesis of g-C₃N₄ electrocatalysts is mainly focused on the construction of catalytic sites and the modulation of the charge transfer kinetics. This work reports a rapid method for synthesizing ultrafine g-C₃N₄ quantum dots (QDs) by electrochemical exfoliation using Al³⁺ ions as an intercalation agent. Uniform g-C₃N₄ QDs with small lateral size and thickness were collected more easily due to the higher charge density and stronger electrostatic force of Al³⁺ ions in the lattice of the host material, compared to conventional univalent alkali cations. The QDs had an average lateral dimension and thickness of 3.5 nm and 1.0 nm, respectively, as determined by TEM and AFM measurements. The presence of a large number of C/N defects was verified by the UV-vis spectra. The ultrafine g-C₃N₄ QDs had a superior hydrogen evolution reaction performance with an ultra-low onset-potential approaching 0 V, and a low overpotential of 208 mV at 10 mA cm⁻², as well as a remarkably low Tafel slope (52 mV·dec⁻¹) in an acidic electrolyte.