Koyama-ji is associated with Yakushi Nyorai, the Medicine Buddha, who is venerated here as the temple's principal deity.

Koyama-ji is in Zentsuji, Kagawa Prefecture, linking it to one of Shikoku's best-known pilgrimage cities.

Young Kobo Daishi played in this area as a child, building mud Buddha statues and plant temples. The field where he played was called Sen-yugahara (Field Where Hermits Play).

A saint appeared from a cave on Mt. Koyama and told Kobo Daishi to build a temple there. That cave, located to the left of the Daishido Hall, became the birthplace of Koyamaji Temple.