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The intimate contact of components in a heterojunction photocatalyst is essential in determining the photocatalytic performance. To build a face‐to‐face interface in a 2D–2D heterojunction is an effective strategy to improve the charge carrier separation and utilization efficiency. In this work, the fabrication of a Step‐scheme heterojunction on thin titania (TiO2) nanosheets with few‐layered MoO3 structures is reported. With a decoration of a low dose of MoO3 layer by ball milling method, TiO2 (B) shows a three‐fold increase in the hydrogen evolution rate. The reaction mechanism and driving force of charge transfer in the S‐scheme heterojunction photocatalyst are investigated and discussed. Such 2D‐2D heterojunction improves photocatalytic reaction rate because photogenerated electrons and holes at the interface can be quickly transferred to the surface. In situ XPS and PL demonstrates that the presence of MoO3 promotes the electron‐hole pair separation via the Step‐scheme heterojunction. Temperature programmed desorption of hydrogen shows that the heterojunction structure facilitates the desorption of hydrogen molecules. The approach used in this work to fabricate 2D–2D assembled structure thus improve the photocatalytic activity can be applied to other 2D metal oxides and sulfides, among others. [ABSTRACT FROM AUTHOR]