Abstract In the presence of Hg2+, a fluorescence resonance energy transfer (FRET) system was constructed between CdSe quantum dots (QDs) (donor) and g-C3N4 (receptors). Nanocomposites of g-C3N4 supported by CdSe QDs (CdSe QDs/g-C3N4 nanosheets) were fabricated through an electrostatic interaction route in an aqueous solution. The nanocomposites were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and transmission electron microscopy. Results showed that the g-C3N4 nanosheets were decorated randomly by CdSe QDs, with average diameter of approximately 7 nm. The feasibility of the FRET system as a sensor was demonstrated by Hg (II) detection in water. At pH 7, a linear relationship was observed between the fluorescence intensity and the concentration of Hg (II) (0–32 nmol/L), with a detection limit of 5.3 nmol/L. The new detection method was proven to be sensitive for detecting Hg2+ in water solutions. Moreover, the method showed high selectivity for Hg2+ over several metal ions, including Na+, Mg2+, Ca2+, Pb2+, Cr3+, Cd2+, Zn2+, and Cu2+. The CdSe QDs/g-C3N4 nanosheet conjugate exhibited desirable long-term stability and reversibility as a novel FRET sensor. The novel FRET-based fluorescence detection provided an attractive assay platform for quantifying Hg2+ in complex water solutions.