Modifying TiO2 to design new photocatalysts with visible light absorption and reduced charge carrier recombination for photocatalytic depollution or water splitting is a very active field. A promising approach is to deposit small nanoclusters of a metal oxide on a semiconducting oxide such as TiO2 or ZnGa2O4. In this paper we present a first principles density functional theory (DFT) investigation of a novel concept in photocatalyst materials design: Sn(II)O nanoclusters supported on TiO2 anatase (001) and demonstrate that the presence of the Sn(II)–O lone pair in the nanoclusters gives a new approach to engineering key properties for photocatalysis. The modification of anatase with Sn(II)O reduces the band gap over unmodified anatase, thus activating the material to visible light. This arises from the upwards shift of the valence band, due to the presence of the Sn 5s–O 2p lone pair in the nanocluster. Enhanced charge separation, which is key for photocatalytic efficiency, arises from the separation of electrons and holes onto the anatase surface and the Sn(II)O nanocluster. This work realises a new strategy of exploiting the lone pair in elements such as Sn to raise the VB edge of modified TiO2 and enhance charge separation in new photocatalyst materials.