Highly crystalline graphitic carbon nitride (g‐C3N4) with decreased structural imperfections benefits from the suppression of electron–hole recombination, which enhances its hydrogen generation activity. However, producing such g‐C3N4 materials by conventional heating in an electric furnace has proven challenging. Herein, we report on the synthesis of high‐quality g‐C3N4 with reduced structural defects by judiciously combining the implementation of melamine–cyanuric acid (MCA) supramolecular aggregates and microwave‐assisted thermolysis. The g‐C3N4 material produced after optimizing the microwave reaction time can effectively generate H2 under visible‐light irradiation. The highest H2 evolution rate achieved was 40.5 μmol h−1, which is two times higher than that of a g‐C3N4 sample prepared by thermal polycondensation of the same supramolecular aggregates in an electric furnace. The microwave‐assisted thermolysis strategy is simple, rapid, and robust, thereby providing a promising route for the synthesis of high‐efficiency g‐C3N4 photocatalysts. Highly crystalline graphitic carbon nitride (g‐C3N4) was synthesized within 16 min by microwave‐assisted thermolysis of melamine–cyanuric acid supramolecular aggregates. The H2 generation rate achieved with the as‐obtained g‐C3N4 material is two times higher than that of a sample prepared by heating the same supramolecular aggregates in a furnace at 540 °C for 2 h.