A metastable intermediate structure (MIS) is identified at the interface between anatase and rutile TiO₂, which enhances the photocatalytic efficiency of P25, a mixture of the two phases. First-principles calculations reveal that the MIS forms due to interfacial confinement and has a high conduction-band minimum (CBM), which increases the overpotential for hydrogen evolution. The MIS also leads to efficient separation of electrons and holes and a wide distribution of the band gap near the interface, improving optical absorption. These factors collectively enhance the photocatalytic performance of P25. The MIS is an intermediate phase along the anatase-to-rutile phase transition, stabilized by interface constraints. The unique band alignment at the interface enables efficient charge separation and improves the reduction ability of hydrogen ions, contributing to the enhanced photocatalytic efficiency. The study provides insights into the mechanism behind the superior performance of P25 and demonstrates a strategy for achieving efficient photocatalysis through interface engineering. The findings highlight the importance of interface structures in determining the properties of mixed-phase materials and suggest that similar interface-confined metastable states may exist in other materials, offering new avenues for material design and application.A metastable intermediate structure (MIS) is identified at the interface between anatase and rutile TiO₂, which enhances the photocatalytic efficiency of P25, a mixture of the two phases. First-principles calculations reveal that the MIS forms due to interfacial confinement and has a high conduction-band minimum (CBM), which increases the overpotential for hydrogen evolution. The MIS also leads to efficient separation of electrons and holes and a wide distribution of the band gap near the interface, improving optical absorption. These factors collectively enhance the photocatalytic performance of P25. The MIS is an intermediate phase along the anatase-to-rutile phase transition, stabilized by interface constraints. The unique band alignment at the interface enables efficient charge separation and improves the reduction ability of hydrogen ions, contributing to the enhanced photocatalytic efficiency. The study provides insights into the mechanism behind the superior performance of P25 and demonstrates a strategy for achieving efficient photocatalysis through interface engineering. The findings highlight the importance of interface structures in determining the properties of mixed-phase materials and suggest that similar interface-confined metastable states may exist in other materials, offering new avenues for material design and application.