A zero-dimensional/two-dimensional heterostructure consists of binary SnO₂–ZnO quantum dots (QDs) deposited on the surface of graphitic carbon nitride (g-C₃N₄) nanosheets. The so-called SnO₂–ZnO QDs/g-C₃N₄ hybrid was successfully synthesized via an in situ co-pyrolysis approach to achieve efficient photoactivity for the degradation of pollutants and production of hydrogen (H₂) under visible-light irradiation. High-resolution transmission electron microscopy images show the close contacts between SnO₂–ZnO QDs with the g-C₃N₄ in the ternary SnO₂–ZnO QDs/g-C₃N₄ hybrid. The optimized hybrid shows excellent photocatalytic efficiency, achieving 99% rhodamine B dye degradation in 60 min under visible-light irradiation. The enriched charge-carrier separation and transportation in the SnO₂–ZnO QDs/g-C₃N₄ hybrid was determined based on electrochemical impedance and photocurrent analyses. This remarkable photoactivity is ascribed to the “smart” heterostructure, which yields numerous benefits, such as visible-light-driven fast electron and hole transfer, due to the strong interaction between the SnO₂–ZnO QDs with the g-C₃N₄ matrix. In addition, the SnO₂–ZnO QDs/g-C₃N₄ hybrid demonstrated a high rate of hydrogen production (13 673.61 μmol g^–1), which is 1.06 and 2.27 times higher than that of the binary ZnO/g-C₃N₄ hybrid (12 785.54 μmol g^–1) and pristine g-C₃N₄ photocatalyst (6017.72 μmol g^–1). The synergistic effect of increased visible absorption and diminished recombination results in enhanced performance of the as-synthesized tin oxide- and zinc oxide-modified g-C₃N₄. We conclude that the present ternary SnO₂–ZnO QDs/g-C₃N₄ hybrid is a promising electrode material for H₂ production and photoelectrochemical cells.