A two-dimensional cyanide-bridged Co–W bimetal assembly, (H₅O₂⁺)[Co(4-bromopyridine)₂{W(CN)₈}], was prepared. A synchrotron radiation (SR) X-ray single-crystal measurement shows that the crystal structure is monoclinic in the P2₁/c space group. Magnetic and spectroscopic measurements show that this assembly takes Co^III_low-spin(S = 0)–W^IV(S = 0) in the temperature range of 2–390 K. Such a wide temperature range Co^III_low-spin–W^IV phase has not been reported so far. First-principles calculations show that the band gap is composed of a W^IV valence band and a Co^III conduction band. 785 nm light irradiation causes photo-induced magnetization with a Curie temperature of 27 K and a coercive field of 2000 Oe. The crystal structure of the photo-induced phase was determined to have larger lattice constants in the two-dimensional layer (bc-plane) by 3% compared to the original phase, which is due to the expansion of the distance of Co–N. The photo-induced phase returns to the original phase upon thermal treatment. First-principles calculations, and magnetic, and optical measurements prove that this photomagnetism is caused by the optical charge-transfer-induced spin transition from Co^III_low-spin(S = 0)–W^IV(S = 0) to Co^II_high-spin(S = 3/2)–W^V(S = 1/2).