Size- and shape-tunable colloidal semiconductor nanocrystals are among the most promising materials for photoelectrochemical water splitting. However, in-depth insights into dimension-dependent charge carrier separation and transport for colloidal semiconductor NCs are still lacking in the contemporary literature. Herein, we experimentally compared photoelectrochemical performance of heavy-metal-free ZnSe nanodots and nanorods with the same cubic structure (zinc blende), similar volumes, and similar absorption edge positions and performed density functional theory (DFT) calculations to study the correlation between the dimension and the electronic structures of ZnSe dots and rods. To eliminate the influence of the different deposition amount of NRs and NDs on each phtoanode, we quantified an average photocurrent density contribution of each single ZnSe dot and rod to be 5 × 10-12 and 9 × 10-12 μA·cm-2, respectively, which highlights a significant PEC performance enhancement of 80% for rods versus dots. DFT calculations have shown that the one-dimensional morphology and crystal plane orientation (⟨111⟩) are both major factors for extremely high transition dipole moment density, which facilitate the charge carrier separation and mobility for ZnSe nanocrystals of different dimensions. This work provides useful insights into the mechanism of photoelectrochemical performance enhancement of colloidal nanocrystals and is beneficial for the design of semiconductor materials for optimal photoelectrochemical cells.