In this paper, a novel hybrid photocatalyst comprising of CdSe quantum dots (QDs) supported on thiol (-SH) functionalized g-C3N4 sheet (TF-g-C3N4) has been synthesized and studied for hydrogen (H-2) evolution. Thiol-functional group has a strong affinity towards the CdSe QDs, which assists the uniform dispersion of 0-dimensional CdSe QDs on TF-g-C3N4 sheets. The hybrid-structure of CdSe TF-g-C3N4 prepared by different weight % loading of CdSe QDs (6 wt %, 12 wt %, 18 wt % and 24 wt %) on TF-g-C3N4 sheets. Thiol-functional group on the g-C3N4 sheet acts as a hole quencher, which suppresses the photogenerated charge recombination. The enhanced photocatalytic rate of H-2 generation was observed for the functionalized TF-g-C3N4 sheet (8.1 times) as compared to non-functionalized g-C3N4 sheet. The hybrid-structure photocatalyst with 18 wt% CdSe QDs on TF-g-C3N4 sheet shows the synergetic increase in photocatalytic H-2 evolution compared to its components due to intimate interfacial attachment between CdSe QDs and -SH functional group. At an irradiation wavelength of lambda >= 420 nm, 18 wt% CdSe-TF-g-C3N4 shows steady H-2 evolution at the rate of 31,000 mu molh(-1) g(-1), which is over 103.3 fold the rate of H-2 evolution for g-C3N4 sheets and 12.3 times TF-g-C3N4. The developed heterostructure photocatalyst 18 wt% CdSe-TF-g-C3N4 shows an outstanding apparent quantum yield (AQY) efficiency of 13.86% at an irradiation wavelength of 460 nm. Hence, this novel heterostructure, CdSe -TF-g-C3N4 proves itself as one of the capable candidates for visible-light photocatalyst with optimum morphology, and bandgap. (C) 2020 Elsevier Ltd. All rights reserved.