A novel Sn(IV) meso-tetraacenaphthylporphyrin (SnTAcP) has been synthesized and characterized. SnTAcP was complexed with methyl-β-CD (mβ-CD), a nanocarrier that enhances water solubility, and the complexes were evaluated as PDT agents using MCF-7 breast cancer cells. A relatively low singlet oxygen quantum yield value of 0.36 was obtained in DMF, and the lowest energy Q band lies at 608 nm on the edge of the therapeutic window. SnTAcP was found to be non-toxic in the dark and phototoxic towards MCF-7 breast cancer cells with a half-maximal inhibitory concentration (IC50) value of 11 ± 1.1 µg.mL−1 after 30 min of irradiation with a 625 nm Thorlabs LED that provides a dose of 432 J.cm−2. A higher IC50 value of 21 ± 1.1 µg.mL−1 was obtained for the mβ-CD inclusion complex of SnTAcP.