A series of graphene-TiO2 nanocomposites (GR-TiO2) with different TiO2 dimensionalities (including TiO2 nanoparticles, nanotubes and nanosheets) and GR weight ratios have been prepared via the analogous in-situ synthesis approach. For the first time, a systematic comparison among GR-TiO2 NP, GR-TiO2 NT and GR-TiO2 NS composite photocatalysts toward ultraviolet-driven CO2 reduction and visible-light-induced benzylamine oxidation has been carried out based on a reasonable benchmark framework. The influences of TiO2 dimensionalities and GR weight ratios on photocatalytic redox reactions are systematically investigated. Moreover, direct comparisons among GR-TiO2 NP, GR-TiO2 NT and GR-TiO2 NS composites are also conducted in terms of reactive species generation, transient photocurrent response and interfacial charge-transfer kinetics. Due to intimate interfacial contact between graphene and two-dimensional (2D) TiO2 nanosheets, 2D-2D GR-TiO2 NS nanocomposites possess superior electronic coupling compared to 2D-0D GR-TiO2 NP and 2D-1D GR-TiO2 NT samples, which results in rapid photoinduced charge-transfer and remarkably improved photocatalytic activity for CO2 reduction under ultraviolet irradiation. On the other hand, because of large BET surface area, GR-TiO2 NT composites possess stronger adsorption for benzylamine and the resulting visible-light absorption, leading to greater photocatalytic activity toward selective oxidation of benzylamine under visible light illumination. Overall, the results suggest that TiO2 nanomaterial dimensionality plays the key role in determining the interfacial charge-transfer and reaction specificity of GR-TiO2 composite photocatalysts.