Sonodynamic therapy (SDT) is a novel promising approach for the minimally invasive treatment of cancer derived from photodynamic therapy (PDT). In this study, we have explored an effective sonosensitizer for SDT by loading the iridium(III) complex [Ir(ppy)2(en)] OOCCH3, where ppy = 2-phenylpyridine and en = ethylenediamine], from now on referred to as Ir, with high photosensitizing ability, into echogenic nanobubbles (Ir-NBs). Akin to photosensitizers, sonosensitizers are acoustically activated by deep-tissue-penetrating low-frequency ultrasound (US) resulting in a localized therapeutic effect attributed to an excessive generation of reactive oxygen species (ROS). The Ir-NB formulation was optimized, and the in vitro characterizations were carried out, including physical properties, acoustic performance, intracellular ROS generation, and cytotoxicity against two human cancer cell lines. Ir-NBs had an average size of 303.3 ± 91.7 nm with a bubble concentration of 9.28 × 1010 particles/mL immediately following production. We found that the initial Ir feeding concentration had a negligible effect on the NB size, but affected the bubble concentration as well as the acoustic performance of the NBs. Through a combination of sonication and Ir-NBs treatment, an increase of 68.8% and 69.6% cytotoxicity in human ovarian cancer cells (OVCAR-3) and human breast cancer cells (MCF-7), respectively, was observed compared to the application of Ir-NBs alone. Furthermore, Ir-NBs exposed to the US also induced the highest levels of intracellular ROS generation compared to free Ir and free Ir with empty NBs. The combination of these results suggests that the differences in treatment efficacy is a direct result of acoustic cavitation. These results provide evidence that US activated Ir-loaded NBs have the potential to become an effective sonosensitizer for SDT.