The adsorption of multiple hydrogen molecules on first-row transition metals (M = Sc–Zn) decorated on heptazine-based graphitic carbon nitride nanosheet/graphene (hg‒C3N4‒NS/G) nanocomposites was investigated using density functional theory (DFT) with the PBE-D3 method. The single H2 adsorption on V-, Cr-, Mn-, Co-, Ni-, and Zn-decorated hg‒C3N4‒NS/G surfaces is characterized by weak physisorption, of which adsorption energies are within the range of 􀀀 0.09 to 􀀀 0.19 eV. In contrast, Sc-, Ti-, Fe-, and Cu-decorated nanocomposites exhibit significantly stronger H2 adsorption (􀀀 0.33 to 􀀀 0.54 eV), driven by Kubas-type interactions, fully meeting DOE requirements. Further evaluation of multiple H2 adsorption on these four systems indicates moderate physisorption, of which adsorption energies are within the range of 􀀀 0.08 to 􀀀 0.10 eV. These findings suggest that Sc-, Ti-, Fe-, and Cu-decorated hg‒C3N4‒NS/G nanocomposites are promising candidates for practical hydrogen storage and provide a basis of rational design of high-performance nanostructured materials for hydrogen storage.