CH3OH on a single-crystal rutile TiO2(110) surface is a widely studied model system for heterogeneous photocatalysis. Using spin-polarized density functional theory with a hybrid functional (HSE06), we study the photocatalytic oxidation of CH3OH adsorbed at a coordinately unsaturated Ti site as an excited-state process with triplet spin multiplicity. The oxidation to CH2O is stepwise and involves a CH3O intermediate. The first O-H dissociation step follows an excitonic interfacial proton-coupled electron transfer mechanism where the hole-electron (h-e) pair generated during the excitation is bound, and the h is transferred to the adsorbate. The O-H dissociation paths associated with other he pairs are unreactive, and the moderate experimental efficiency is due to the different reactivity of the h-e pairs. The excited-state CH3O intermediate further deactivates through a seam of intersection between the ground and excited states. It can follow three different paths, regeneration of adsorbed CH3OH or formation of the ground-state CH3O anion or an adsorbed CH2O radical anion. The third channel corresponds to photochemical CH2O formation from CH3OH, where a single photon induces one electron oxidation and transfer of two protons. These results expand the current view on the photocatalysis of CH3OH on TiO2(110) by highlighting the role of excitons and showing that adsorbed CH3OH may also be an active species in the photocatalytic oxidation to CH2O.