Vitiligo is a dermatological autoimmune disease characterized by the loss of melanin pigment. The pathogenesis of vitiligo involves oxidative stress and the autoimmune response mediated by it. Given the current state of vitiligo treatment and the role of tetrahydrobiopterin (H4Bip) as a trigger of vitiligo, the purpose of this work is to study the effect of Pt-Pd nanoparticles (NPs) on the auto- and photooxidation H4Bip as a therapy for vitiligo. Electron microscopy determined: the diameter of palladium NPs was 3.59 ± 0.56 nm and the diameter of platinum NPs was 1.93 ± 0.34 nm. The zeta potential was −16.9 ± 3.4 mV. It was demonstrated that Pt-Pd NPs catalyze the autoxidation of H4Bip to dihydropterins and their catalytic activity is proportional to the Pt-Pd NPs concentration. The therapeutic effect of NPs may be due to the fact that dihydropterins do not inhibit melanogenesis. NPs also catalyze the formation of dihydropterin dimers upon irradiation of H4Bip solutions at 308 and 325 nm. The removal of H4Bip by photooxidation is apparently the basis for phototherapy of vitiligo. We have shown for the first time that, along with catalase-like and superoxide dismutase-like effects, one of the main pathways of action of Pt-Pd NPs is the catalysis of the oxidation of H4Bip to dihydropterins and their conversion into dimers under UV irradiation in situ. Thus, Pt-Pd NPs are apparently able to remove excess of H4Bip and thereby prevent the development of strong oxidative stress leading to the development of vitiligo.