Molecular photoabsorption enhancement under ambient solar radiations can improve efficiency substantially in renewable energy production. Here, we explore the theoretical basis and experimental evidences that nanostructured molecular assemblies exhibit an unprecedented property of synergistic photon absorption enhancement. The molecular mechanism of this enhancement phenomenon originates from the combined effect of the photon-molecule interaction and the electronic energy transfer between two adjacent molecular assemblies. For a natural system, the synergistic photon absorption enhancement factor of green algae (Chlorella vulgaris) in vivo at 632.8 nm was determined to be 103. This enhanced photon absorption process in nanostructured molecular assemblies opens a doorway to create entangled double excitons by incoherent solar radiations.