An unusual photochemistry of water-assisted self-photoredox of 3-(hydroxymethyl) benzophenone 1 has been investigated by CASPT2//CASSCF computations. The water-assisted self-photoredox is found to proceed via three sequential reactions: an excited-state intermolecular proton transfer (ESIPT), a photoinduced deprotonation, and a self-redox reaction. Upon photoexcitation at 243 nm, the system of 1 is taken to the Franck-Condon region of a short-distance charge transfer (SCT) state of S(SCT)((1)ππ*) and then undergoes ESIPT with a small barrier of ∼3.4 kcal/mol producing the intermediate 2. Subsequently, the singlet-triplet crossing (STC) of STC ((1)ππ*/(3)ππ*) relays 2 by intersystem crossing to the T(SCT)((3)ππ*) state followed by a deprotonation reaction overcoming a moderate barrier of ∼8.0 kcal/mol and finally produces the triplet biradical intermediate 3. Another moderate barrier (∼5.8 kcal/mol) in the T(SCT)((3)ππ*) state has to be overcome so as to relax to a second singlet-triplet crossing STC(T/S0) that allows an efficient spin-forbidden decay to the ground state. The self-redox reaction aided by water molecules occurs with tiny barriers in the S0 state via two steps, protonation of the benzhydrol carbon to produce intermediate 4 and then deprotonation from the benzylic oxygen to yield the final product 3-formylbenzhydrol 5.