Photocatalysis involving oxygenated-intermediate conversion is rather difficult to achieve over main-group metals owing to the inherent absence of empty and filled host d orbitals. Nevertheless, main-group metals (e.g., Mg) as cofactors within enzymes are highly active in biochemical reaction processes. By theoretical predictions and in situ diffuse reflectance infrared Fourier transform spectroscopy, we confirm that tri-s-triazine-based N-coordinated Mg cofactors can be activated to efficiently complete photocatalytic H 2 O dissociation to H 2 and O 2 . Photogenerated e − and h + in the spontaneously formed localized electric field caused by increasing entropy realize the separation in space and migration in the direction to N and single Mg atoms to dehydrogenate and subsequently deoxidize the adjacent hydroxyl group intermediates for H 2 and O 2 generation, respectively, with a solar-to-H 2 efficiency of 0.88% under AM 1.5 G excitation. This work provides a perspective to activate main-group metals and deeply understand their catalytic mechanism of the artificial photocatalytic H 2 O overall splitting process.