A plasmonic photocatalyst of Ag/AgCl@TiO2 nanoparticles (NPs) was directly prepared by a one-step sonochemical method. Both Ag NPs and AgCl were co-deposited on TiO2 NPs to form Ag@TiO2 and Ag/AgCl@TiO2 using the method. Due to the localized surface plasmonic effects of Ag NPs, the visible-light absorbance of the Ag/AgCl@TiO2 photocatalyst was dramatically increased and the photocatalytic activity to decompose Rhodamine B was much improved under visible light. In addition, due to the advantages of the sonochemical approach, only a very small amount of Ag is required to obtain a high photocatalytic activity in the plasmonic catalyst. The mechanism for the enhancement of the visible-light-driven photocatalytic activities was also analyzed.