A photocatalyst TiO2/Ti‐BPDC‐Pt is developed with a self‐grown TiO2/Ti‐metal–organic framework (MOF) heterojunction, i.e. TiO2/Ti‐BPDC, and selectively anchored high‐density Pt single‐atomic cocatalysts on Ti‐BPDC for photocatalytic hydrogen evolution. This intimate heterojunction, growing from the surface pyrolytic reconstruction of Ti‐BPDC, works in a direct Z‐scheme, efficiently separating electrons and holes. Pt is selectively anchored on Ti‐BPDC by ligands and is found in the form of single atoms with loading up to 1.8 wt %. The selective location of Pt is the electron‐enriched domain of the heterojunction, which further enhances the utilization of the separated electrons. This tailored TiO2/Ti‐BPDC‐Pt shows a significantly enhanced activity of 12.4 mmol g−1 h−1 compared to other TiO2‐ or MOF‐based catalysts. The structure‐activity relationship further proves the balance of two simultaneously exposed domains of heterojunctions is critical to fulfilling this kind of catalyst. [ABSTRACT FROM AUTHOR]