Gallium arsenide (GaAs) heterojunctions have been widely explored for their promising applications in solar cells (SCs) and photoelectrochemical (PEC) water splitting, owing to their cost-effective design and great potential for enhancing power conversion efficiency (PCE). In this study, an innovative MoS₂ hole-transport layer was introduced into the GaAs heterojunction for applications in SCs and PEC water splitting. By optimizing the thickness of the MoS₂ film, the sulfide oxidation reaction at the heterointerface was effectively suppressed. Significantly, a synergistic system integrating a GaAs heterojunction SC with a photoelectrode was proposed. The incorporation of carbon nanotubes into the GaAs/MoS₂ heterojunction significantly improved charge carrier transport, enhancing the PCE from 0.24% to 12.41%. In the PEC water splitting system, the GaAs/MoS₂ heterostructure also demonstrated excellent oxygen evolution reaction performance. This optimization led to a maximum applied bias photon-to-current efficiency of 35% under bias, reaching 20% at 0 V vs. reversible hydrogen electrode (RHE), along with a photocurrent density of 40 mA cm^-2 and a solar-to-hydrogen conversion efficiency of 17.22%. When integrated into a photovoltaic-PEC system, the GaAs/MoS₂ photoelectrode achieved a current density of 20 mA cm^-2 at 0 V vs. RHE, with a 400 mV negative shift in the water oxidation onset potential, enabling highly efficient solar-driven hydrogen production.