Photocatalytic water splitting, a process aimed at hydrogen production, is considered a pivotal clean energy sources to achieve environmental friendliness and high energy efficiency in the future. The crux of achieving this objective lies in the design of stable and efficient catalysts. Herein, we proposed a novel type-II band alignment GaSe/YAlS van der Waals heterostructure with robust stability for photocatalytic water splitting. The intrinsic electric field of YAlS monolayer can break the requirement of band gap (>1.23 eV) in traditional photocatalytic water splitting theory and significantly enhancing the utilization of solar energy. Encouragingly, compared to individual monolayers GaSe or YAlS, the optical absorption coefficients of the GaSe/YAlS heterostructure are significantly improved and the corrected solar-to-hydrogen (STH) efficiency is improved by 406% and 132%, respectively, reaching an impressive 29.73% efficiency. Furthermore, the heterostructure demonstrates exceptional overall water splitting performance across a broad pH range and maintains a consistently high STH efficiency (25.41%∼34.71%) under a wide range biaxial strain (−6%–5%). In addition, the designed GaSe/YAlS heterostructure showcases enhanced absorption of visible as well as partial absorption of infrared light for photocatalytic water splitting. These compelling findings unequivocally establish GaSe/YAlS heterostructure as an excellent and promising candidate for photocatalytic water splitting.