By late medieval times, it was common to describe Prince Shōtoku as “Japan’s Śākyamuni,” but how was he symbolically elevated to this status? This article considers “personalized relics”—unique remains identified as a particular part of the actual body of an honored figure—and shows how they played a crucial role in transforming the sacred identities of the Japanese and South Asian princes. The three sections each deal with a different type of relic associated with Shōtoku: a miraculously manifested eye, locks of hair, and various teeth that were enshrined, stolen, and re-enshrined. These case studies trace the incremental replacement of Śākyamuni with Shōtoku as a source of religious power and authority, one that surpassed its original model for its unique and inalienable connection to devotees in medieval Japan.