This is the first study to demonstrate that Microcystis aeruginosa, a typical algal bloom-forming cyanobacterium, can be effectively inactivated by ultrasound (US) irradiation in the presence of titanium dioxide (TiO2) particles as a sonocatalyst. When a culture broth of M. aeruginosa was ultrasonically irradiated for 15 min in the presence of 0.5 g/mL of TiO2 particles 2 mm in diameter, the cell survival ratio was 0.13, which was significantly lower than that in the case of US irradiation alone (0.87). Moreover, regrowth of M. aeruginosa in the culture was also inhibited for 10 days following ultrasonic disinfection in the presence of TiO2 particles for 15 min.