The introduction of single layer carbon nitride (SLCN) sheets into a colloidal ZnO solution in DMSO results in the termination of the growth of ZnO nanocrystals (NCs) and formation of binary colloidal ZnO/SLCN heterostructures with an average ZnO core size tunable between 3.0 and 5.5 nm. The ZnO/SLCN heterostructures reveal two photoluminescence (PL) bands corresponding to the ZnO core (a band peaked at 490–560 nm depending on the ZnO core size) and SLCN shell (a band peaked at 350 nm). The combination of SLCN and ZnO NCs into a heterostructure results in a reduction of the SLCN PL lifetime <τ> from 860 ns to 3.5–6.6 ns indicating the possibility of interfacial charge transfer from the photoexcited SLCN to the ZnO core NCs. At the same time, the <τ> of ZnO cores is also reduced from 0.70 to 1.49 μs for the core ZnO NCs with a “passive” SiO2 shell to 76–400 ns for the ZnO/SLCN heterostructures, indicating the possibility of photoinduced charge transfer from the ZnO core to the SLCN shell. The rate constant of such charge transfer was found to be 10.5 × 106–12.0 × 106 s−1 for the 3.0–3.6-nm ZnO cores and decreasing to 2.7 × 106 s−1 for 4.0-nm ZnO cores and to 1.1 × 106 s−1 for the 5.5-nm cores. The size dependence is caused by a decrease of the gap between a donor conduction band level of the ZnO cores and an acceptor conduction band level of the SLCN sheets. The ZnO/SLCN NCs also reveal a size-dependent capability of photoinduced charging under continuous UV excitation.