Herein we report on the study of novel dinuclear ruthenium(II) complexes designed to target and to photo-react with G-quadruplex telomeric DNA. Upon irradiation, complexes efficiently generate guanine radical cation sites as photo-oxidation products. The compounds also display efficient cell penetration with localization to the nucleus and show strong photocytotoxicity toward osteosarcoma cells. Thanks to a microscopic-based telomere dysfunction assay, which allows the direct visualization of DNA damage in cells, we brought the first evidence of forming photo-oxidative damage at telomeres in cellulo. This emphasizes interesting prospects for the development of future cancer phototherapies.