Abstract BACKGROUND C 3 N 4 nanosheets with a layered structure have attracted significant interest in the field of photocatalysis. However, direct preparation of these nanosheets remains a significant challenge. RESULTS In this study, highly active C 3 N 4 nanosheets were fabricated via the direct thermal polymerization of melamine supramolecular aggregates obtained using hydrochloric‐acid‐induced hydrothermal pretreatment. Various methods were adopted to characterize the photocatalytic materials obtained and density functional theory calculations were performed to understand the highly efficient nature of C 3 N 4 . The structures and photochemistry of the photocatalytic materials depend on the hydrochloric‐acid‐induced hydrothermal pretreatment method. It was found that C 3 N 4 ‐180‐H nanosheets have numerous (C) 2 N–H that are conducive to photogenerated electron–hole separation. In addition, the C 3 N 4 ‐180‐H nanosheets have a large surface area and a short charge‐carrier diffusion distance. Furthermore, the C 3 N 4 ‐180‐H nanosheets exhibited a longer excitation‐state charge‐carrier lifetime than bulk C 3 N 4 . The as‐prepared C 3 N 4 ‐180‐H nanosheets with unique structural advantages showed superior photocatalytic H 2 evolution performance (413 μmol h −1  g −1 ) compared to that of bulk C 3 N 4 (90.5 μmol h −1  g −1 ). CONCLUSION This study offers a feasible and simple method for synthesizing C 3 N 4 nanosheets and developing an excellent 2D photocatalyst for superior solar‐to‐H 2 production. © 2023 Society of Chemical Industry (SCI).