This work demonstrates photocatalytic CO2 reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4'-{Et2O3PCH2}2-2,2'-bipyridyl)(CO)3] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO2 to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl4 (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO2 reduction. The incorporation of -P(O)(OEt)2 groups, decoupled from the core of the catalyst by a -CH2- spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by 1H and 13C{1H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.