Photodynamic therapy delivers more targeted cell killing than classical chemotherapy. It uses light-absorbing compounds, photosensitizers (PSs), to generate lethal reactive oxygen species (ROS) at sites of localized irradiation. Transition metal complexes are attractive PSs due to their photostability, visible-light absorption, and high ROS yields. Here, we introduce a low-molecular weight, photostable iridium complex, [Ir(thpy)2(benz)]Cl, 1, that localizes to the Golgi apparatus, mitochondria, and endoplasmic reticulum, absorbs visible light, phosphoresces strongly, generates 1O2 with 43% yield, and undergoes cellular elimination after 24 h. 1 shows low dark toxicity and under remarkably low doses (3 min, 20-30 mJ s-1 cm-2) of 405 or 455 nm light, it causes killing of bladder (EJ), malignant melanoma (A375), and oropharyngeal (OPSCC72) cancer cells, with high phototoxic indices > 100-378. 1 is also an efficient PS in 3D melanoma spheroids, with repeated short-time irradiation causing cumulative killing.