We evaluate poly (d,l-lactide-co-glycolide) (PLGA) nanoparticles embedding inorganic molybdenum octahedral cluster for photodynamic therapy of cancer (PDT). Tetrabutyl ammonium salt of Mo6Br14 cluster unit, (TBA)2Mo6Br14, presents promising photosensitization activity in the destruction of targeted cancer cells. Stable cluster loaded nanoparticles (CNPs) were prepared by solvent displacement method showing spherical shapes, zeta potential values around −30 mV, polydispersity index lower than 0.2 and sizes around 100 nm. FT-IR and DSC analysis revealed the lack of strong chemical interaction between the cluster and the polymer within the nanoparticles. In vitro release study showed that (TBA)2Mo6Br14 was totally dissolved in 20 min, while CNPs were able to control the release of encapsulated cluster. In vitro cellular viability studies conducted on A2780 ovarian cancer cell line treated up to 72 h with cluster or CNPs did not show any sign of toxicity in concentrations up to 20 μg/ml. This concentration was selected for photo-activation test on A2780 cells and CNPs were able to generate oxygen singlet resulting in a decrease of the cellular viability up to 50%, respectively compared to non-activated conditions. This work presents (TBA)2Mo6Br14 as a novel photosensitizer for PDT and suggests PLGA nanoparticles as an efficient delivery system intended for tumor targeting.