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[Display omitted] • g-C 3 N 4 nanosheets were prepared via a molten salts method. • Crystalline g-C 3 N 4 nanosheets was composed of triazine ring unit. • The g-C 3 N 4 nanosheets show the excellent photocatalytic activity for the degradation of RhB. The crystalline g-C 3 N 4 nanosheets have been successfully prepared by one-step molten salts method. The microstructures and optical properties of as-prepared samples were characterized by different techniques, such as XRD, EDS, XPS, SEM, TEM, BET, PL and Uv–vis DRS. The results showed that the unit of the crystalline g-C 3 N 4 nanosheets was composed of triazine ring, which was different from heptazine-based g-C 3 N 4. The crystalline g-C 3 N 4 nanosheets were thin and transparent, which were similar to graphene. The crystalline g-C 3 N 4 nanosheets exhibits the excellent photocatalytic activity, which can degrade 98% RhB within 120 min under visible light irradiation. However, the degradation efficiency of bulk g-C 3 N 4 is only 40% within 120 min. The enhanced photocatalytic activity of g-C 3 N 4 nanosheets can be attributed to 2D nanosheet morphology, which inhibits the electron-hole recombination. In addition, the high specific surface area of g-C 3 N 4 nanosheets positively increases the contact area with pollutants, making the degradation reaction more efficiently. A plausible photocatalytic mechanism was proposed for the degradation of RhB under visible light irradiation. [ABSTRACT FROM AUTHOR]