Band gap engineering on wide band gap semiconductor photocatalysts is of critical significance in improving their solar energy utilization. Here, we have successfully realized large band gap reduction and visible light photocatalytic water splitting on a wide band gap semiconductor Mg4Ta2O9 by nitrogen doping. More than 2.6 eV reductions in band gap have been achieved after doping nitrogen into Mg4Ta2O9, which strikingly red-shifts its absorption edge from less than 300 nm to as far as 600 nm. The nitrogen doped Mg4Ta2O9, i.e. Mg4Ta2O9Ny shows promising photocatalytic activities for water reduction and oxidation under visible light illumination (lambda >= 400 nm). Apparent quantum efficiency as high as similar to 1.57% has been achieved at 420 +/- 20 nm which is comparable to a number of active metal oxynitride photocatalysts. Photoelectrochemical analysis suggests that this band gap reduction stems mainly from an uplift of valence band edge position along with nitrogen doping. Our band gap engineering on a compact corundum type compound implies that nitrogen doping can be applicable to a wider range of metal oxides, not necessarily limited to those with an 'open' crystal structure (tunnels or layers, etc.) reported in the literatures, thereby opening new possibilities in searching and developing visible light active photocatalysts. (C) 2019 Elsevier Inc. All rights reserved.