The search for efficient and stable photocatalysts for solar−driven water splitting remains a critical challenge in renewable energy research. This study investigates the B2CSe/Mg(OH)2 van der Waals heterostructure (vdWH) as a promising candidate using first−principles simulations. The heterostructure demonstrates exceptional thermal, kinetic, and mechanical stability, as confirmed by ab initio molecular dynamics, phonon dispersion, and mechanical property analyses. The B2CSe/Mg(OH)2 vdWH exhibits a reduced indirect bandgap compared to the Mg(OH)2 monolayer, facilitating efficient photogenerated electron−hole pair separation. A type−II band alignment, supported by charge density difference, electronic structure, and built−in electric field analyses, further enhances redox capacity and carrier separation efficiency. The heterostructure achieves a remarkable solar−to−hydrogen (STH) conversion efficiency of 34.58%, outperforming many existing systems, and demonstrates strong optical absorption across the visible light spectrum. Strain engineering reveals the potential for adaptive photocatalyst design, with compressive strain inducing a transition from type-II to type-I band alignment, and tensile strain effectively redshifting the absorption edge to harness a broader range of solar energy. This tunability allows for precise control over the electronic and optical properties of the heterostructure, enabling optimization for specific photocatalytic applications. A potential drop of 8.06 eV across the interface and a charge transfer of 0.0045 electrons from Mg(OH)₂ to B₂CSe further enhance the heterostructure’s photocatalytic potential. These findings not only highlight the B2CSe/Mg(OH)2 vdWH as a highly efficient and stable photocatalyst for overall water splitting but also underscore the transformative role of strain engineering in designing adaptive photocatalysts. This approach offers a promising pathway for advancing solar energy utilization and hydrogen production, paving the way for next-generation renewable energy technologies.