The activity of many water-splitting photocatalysts could be improved by the use of Rh(III)-Cr(III) mixed oxide (Rh 2- x Cr x O 3 ) particles as cocatalysts. Although further improvement of water-splitting activity could be achieved if the size of the Rh 2- x Cr x O 3 particles was decreased further, it is difficult to load ultrafine (<2 nm) Rh 2- x Cr x O 3 particles onto a photocatalyst using conventional loading methods. In this study, we successfully established a new loading method and used it to load Rh 2- x Cr x O 3 particles with a size of approximately 1.3 nm and a narrow size distribution onto the BaLa 4 Ti 4 O 15 photocatalyst. The obtained photocatalyst exhibited an apparent quantum yield of 16%, which is the highest achieved for BaLa 4 Ti 4 O 15 to date. Thus, the developed loading technique of Rh 2- x Cr x O 3 particles is extremely effective at improving the activity of the water-splitting photocatalyst BaLa 4 Ti 4 O 15 . This method is expected to be extended to other advanced water-splitting photocatalysts to achieve higher quantum yields.