A mechanism of photoreduction of an aliphatic nitro compound — nitromethane — in the presence of a hydrogen donor, dimethylamine, was proposed. Quantum chemical calculations (using the ωB97X-D3/def2-TZVPP method in ORCA) were performed to determine the thermodynamic parameters of the proposed reaction steps. Changes in enthalpy and Gibbs free energy were computed for 12 possible elementary steps, with three key reactions selected for further analysis. Transition states for these key steps were located and reaction energy profiles constructed. The lowest activation energy (6.5 kcal/mol) was found for the hydrogen atom transfer from the methyl group of dimethylamine, while some other stages proceeded without an activation barrier. It was shown that the OH group does not detach directly from the nitrogen atom but through a cascade of rearrangements, including hydroxyl migration. The reaction occurs only for aliphatic nitro compounds due to the presence of an α-hydrogen, which is absent in aromatic analogs. The final mechanism consists of 5 consecutive steps and supports the hypothesis about the specific role of the triplet state of nitromethane in reductive photochemical processes. The study highlights the significance of intramolecular rearrangements in lowering activation barriers. These findings may serve as a basis for further design of light-induced reduction pathways for nitro-containing compounds.