Photosynthesis is a process wherein the chromophores in plants and bacteria absorb light and convert it into chemical energy. To mimic this process, an emissive poly(ethylene glycol)‐decorated tetragonal prismatic platinum(II) cage was prepared and used as the donor molecule to construct a light‐harvesting system in water. Eosin Y was chosen as the acceptor because of its good spectral overlap with that of the metallacage, which is essential for the preparation of light‐harvesting systems. Such a combination showed enhanced catalytic activity in catalyzing the cross‐coupling hydrogen evolution reaction, as compared with eosin Y alone. This study offers a pathway for using the output energy from the light‐harvesting system to mimic the whole photosynthetic process. Harvest lights: An aqueous light‐harvesting system, based on a platinum(II) cage and eosin Y, demonstrates efficient energy transfer. The system showed improved photocatalytic activity in a cross‐coupling hydrogen evolution reaction compared with the reaction employing just eosin Y alone.