Ruthenium(II) polypyridyl complexes exhibit a lack of selectivity toward cancer tissues despite extensive studies as photosensitizers for photodynamic therapy (PDT). Here, we report pH-activatable RuII photosensitizers for molecularly targeted PDT by exploiting the higher acidity of tumoral tissue. The fluorescein moiety, well known for its high pH sensitivity, was connected to a RuII center to yield novel photosensitizers for pH-sensitive 1O2 photogeneration. Their ability to photosensitize molecular dioxygen was studied at various pHs and revealed a drastic enhancement from 0.07 to 0.66 of the 1O2 quantum yield under acidic conditions (pH 7.5 to pH 5.5). Their photocytotoxicity against U2OS osteosarcoma cells was also investigated at pH 5.5 and 7.5 through IC50 determination. A strong enhancement of the photocytotoxicity reaching 930 nM was observed at pH 5.5, which showed the potential of such photosensitizers for pH-activatable PDT.