A new, biocompatible photooxygenation catalyst that can selectively oxygenate and degrade the pathogenic aggregation of Alzheimer’s disease (AD)-related amyloid-β peptide (Aβ) under near-infrared (NIR) light irradiation has been developed. The catalyst was able to oxygenate Aβ embedded under the skin of a living mouse, and diminished intact Aβ level in AD-model mouse brain. The new catalyst is potentially applicable for the treatment of peripheral amyloid diseases and AD.