Hydrogen has attracted increasing attention as clean energy for fuel cells over the past decade. Photoelectrochemical (PEC) water splitting is considered the most feasible production method. However, its practical efficiency depends significantly on the photogeneration rate of electron (e⁻) and hole (h⁺) on a semiconductor photoanode and the rapid separation of these charge carriers. Also, a proper match of small and large bandgap positions is necessary. 3D core-shell heterostructures of WO₃/Bi₂MoO₆/Co‐Pi and BiVO₄/MoS₂ were synthesized using hydrothermal/electrodeposition and electrodeposition/plasma-enhanced atomic layer deposition (PECALD), respectively. The as-prepared WO₃/Bi₂MoO₆/Co‐Pi hetero-photocatalyst exhibited significantly high photoelectrochemical activity, where its photocurrent efficiency was 4.6 times greater than that of the constituent WO₃. For the BiVO₄/MoS₂ photoelectrode, an optimal thickness of the MoS₂ layer was controlled by the number of PEALD cycles. The highest photocurrent (2.1 mA cm^-2) was produced from the electrode with a 6 nm layer-thickness, 2.4 times higher than the pristine BiVO₄ at an external bias of 1 V vs. Ag/AgCl. The appropriate bandgap alignment between layers can corroborate such drastic improvement in the PEC properties. A detailed investigation into the photoelectrochemical reaction mechanism showed that enhancement of the PEC performance was attributed to the finely deposited MoS₂ layer, which expands the spectral absorption edge of solar light and enhances the suppression of electron-hole pair recombination because of the proper alignment match between the two semiconductors with a small and large bandgap. Furthermore, a heterojunction charge-transfer mechanism was proposed to verify the role of the co-catalyst, Co‐Pi, in enhancing the photocurrent at the WO₃/Bi₂MoO₆ photoanode under the same applied bias.