Photocatalytic water splitting holds a key to realizing the hydrogen economy but faces a grand challenge, namely, the severe charge recombination causes low solar-to-hydrogen (STH) efficiency. This work reports a new form of MAPbI3 microcrystal (MAPbI3-MC)/monolayer MoS2 nanosheets (ML-MoS2) composite photocatalyst with type II heterojunction to effectively suppress the charge recombination for efficient photocatalytic hydrogen generation via HI splitting. The ML-MoS2/MAPbI3-MC is made of a large-sized MAPbI3-MC light harvester anchored with electrocatalytically active ML-MoS2 nanosheets to form a type II heterojunction that possesses a strong built-in electric field aligned between the spatially well-defined MAPbI3-MC and ML-MoS2 nanosheets to empower the photocatalyst with superior capability to effectively promote the charge separation. The ML-MoS2/MAPbI3-MC brings the hydrogen generation performance of the perovskite-based photocatalysts to a new horizon with a champion STH efficiency of 1.09% and a record hydrogen generation activity of 13.6 mmol g−1 h−1 under visible light.