The use of dendritic nanoboxes (DBoxes) to protect encapsulated dye molecules against photodegradation was evaluated. We focused on photoinduced oxidation and studied the ability of DBoxes to protect the dyes inside from reactive singlet oxygen. Rubrene and a-terthienyl were used as the probe and generator for singlet oxygen, respectively. Bleaching of encapsulated rubrene in a DBox was 50 times slower than that of rubrene alone. Stability was further improved by attachment of singlet-oxygen-quencher molecules to the surfaces of the DBoxes.