The perovskite yttrium tantalum oxynitride is theoretically proposed as a promising semiconductor for solar water splitting because of the predicted band gap and energy positions of band edges. In experiments, however, we show here that depending on the processing parameters, yttrium tantalum oxynitrides exist in multiphases, including the desired perovskite YTaON₂, defect fluorite YTa(O,N,□)₄, and N-doped YTaO₄. These multiphases have band gaps ranging between 2.13 and 2.31 eV, all responsive to visible light. The N-doped YTaO₄, perovskite main phase, and fluorite main phase derived from crystalline fergusonite oxide precursors exhibit interesting photoelectrochemical performances for water oxidation, while the defect fluorite derived from low-crystallized scheelite-type oxide precursors shows negligible activity. Preliminary measurements show that loading an IrO_x cocatalyst on N-doped YTaO₄ significantly improves its photoelectrochemical performance, encouraging further studies to optimize this new material for solar fuel production.