Novel iridium(III) di-NHC complexes have been synthesized by transmetalation of the dicarbene ligand from the corresponding dicarbene silver complex. The iridium(III) complex [IrClCp*(di-NHC)](PF6) (2; di-NHC = 1,1′-dimethyl-3,3′-ethylenediimidazole-2,2′-diylidene) behaves as an efficient catalyst for the oxidation of water by using CeIV or NaIO4 as sacrificial oxidants, reaching a TOF of 0.20 s–1, comparable with the most active catalysts reported in the literature. Complex 2 also displays remarkable catalytic activity in the photocatalyzed oxidation of water by using [Ru(bpy)3]2+ as sensitizer and persulfate as sacrificial electron acceptor. The fate of the catalyst under turnover conditions was investigated by combined spectroscopic, kinetic and GC–MS methods. In particular, EPR experiments performed under photoirradiation revealed the formation of a high-valent IrIV molecular intermediate involved in the primary photoinduced electron-transfer events.