Efficient utilization of solar energy for the conversion of water into hydrogen via photoelectrochemical (PEC) water splitting holds tremendous promise for sustainable energy production. In this study, we report a novel vertically aligned flake-like CuO/Co3O4 nanoparticle@g-C3N4 sheets ternary nanocomposite as a heterojunction catalyst for enhancing PEC water splitting efficiency. The ternary nanocomposite was synthesized via a facile and scalable method, resulting in a unique structure with vertically aligned CuO/Co3O4 nanoparticles in tight interface interaction with g-C3N4 sheets. The resulting heterojunction exhibited enhanced light absorption, efficient charge separation, and improved charge transfer kinetics, leading to significantly enhanced PEC water splitting performance. The as-synthesized ternary nanocomposite, such as CuO/Co3O4@g-C3N4 nanocomposite, demonstrated superior PEC activity with an enhanced photocurrent density, i.e., 0.88 mA/cm2, which is higher than that of the pristine CuO (0.65 mA/cm2), Co3O4 (0.7 mA/cm2), and CuO-Co3O4 (0.77 mA/cm2) structures with linear sweep voltammetry under light irradiation. Additionally, the ternary nanocomposite exhibited excellent stability during 2 h PEC water splitting measurements under 1 h time interval chopped conditions. The remarkable performance of the vertically aligned CuO/Co3O4 nanoparticle@g-C3N4 ternary nanocomposite underscores its potential as a promising catalyst for efficient solar-driven hydrogen generation. This study not only provides insights into the design of advanced heterojunction catalysts but also contributes to the development of sustainable energy conversion technologies.