Abstract To combat the energy crisis and environmental pollution, developing renewable energy technology such as hydrogen (H 2 ) production is necessary. The sulfur–iodine thermochemical cycle has high commercial potential in conducting hydrogen iodide (HI) splitting for H 2 generation, but it requires high‐temperature conditions. In comparison, photocatalytic HI splitting of halide perovskites is non‐polluted and low‐cost for H 2 production at room temperature. Herein, an in situ constructed multidimensional bismuth (Bi)‐based 3D/2D EDABiI 5 /MA 3 Bi 2 I 9 perovskite heterojunction is developed first by synergistically integrating dimensionality control with heterostructure engineering. Accordingly, the optimal EDABiI 5 /MA 3 Bi 2 I 9 without any co‐catalysts exhibits the H 2 evolution rate of 213.63 µmol h −1 g −1 under irradiation. Equally importantly, interfacial dynamics of solid/solid and solid/liquid interfaces play a crucial role in photocatalytic performance. Therefore, using temperature‐dependent transient photoluminescence and electrochemical voltammetric techniques, it is confirmed that the exciton transportation of EDABiI 5 /MA 3 Bi 2 I 9 is accelerated by stronger electronic coupling arising from an enhanced overlap of electronic wavefunctions. Moreover, the effective diffusion coefficient and electron transfer rate of EDABiI 5 /MA 3 Bi 2 I 9 demonstrate efficient heterogeneous electron transfer, resulting in improved photocatalytic hydrogen production. Consequently, the in situ formation of perovskite heterostructures studied by a combination of photophysical and electrochemical techniques provides new insights into green hydrogen evolution and interfacial interaction dynamics for commercial applications of solar‐to‐fuel technology.