AbstractA highly efficient photocatalytic hydrogen evolution system was prepared by using an octupolar molecule as a building block of self‐assembled nanoparticles. The photocatalytic system showed an enhanced hydrogen evolution reaction (HER) rate upon the addition of halide ions, which was attributed to an external heavy atom effect enhancing intersystem crossing. The HER rate of the photocatalytic system was proportional to the atomic weight and concentration of halide ion additives, and its maximum HER rate reached 460 mmol/g ⋅ h in the presence of iodide ions and a metal co‐catalyst. Further, the photocatalytic system without any halide ion additives generated hydrogen gas utilizing seawater and stimulated sunlight with remarkable efficiency (apparent quantum yield∼3.8 %).