A polymerized complex (PC) technique was utilized to prepare high‐purity barium tetratitanate (BaTi4O9) fine powders at a low temperature (700 °C). BaTi4O9via the PC route, combined with RuO2, exhibited 2.4 times larger photocatalytic activities for the decomposition of water compared to those for a sample prepared by a solid‐state reaction method. A considerably large surface area (∼30 m2/g) of the BaTi4O9/RuO2 powder via the PC route, when compared with ∼5 m2/g for the solid‐state reaction powder, was supposed to be one of the key factors responsible for the high photocatalytic activity observed.