Kongōbu-ji in the context of Mount Kōya

Bhaiṣajyaguru (Sanskrit: भैषज्यगुरु, Chinese: 藥師佛, Japanese: 薬師仏, Korean: 약사불, Vietnamese: Dược Sư Phật, Standard Tibetan: སངས་རྒྱས་སྨན་བླ), or Bhaishajyaguru, formally Bhaiṣajya-guru-vaiḍūrya-prabha-rāja ("Medicine Master and King of Lapis Lazuli Light"; Chinese: 藥師琉璃光(王)如來, Japanese: 薬師瑠璃光如来, Korean: 약사유리광여래, Vietnamese: Dược Sư Lưu Ly Quang Vương Như Lai), is the Buddha of healing and medicine in Mahāyāna Buddhism. Commonly referred to as the "Medicine Buddha", he is described as a doctor who cures suffering (Pali/Sanskrit: dukkha/duḥkha) using the medicine of his teachings.

The image of Bhaiṣajyaguru is usually expressed with a canonical Buddha-like form holding a gallipot and, in some versions, possessing blue or deep green skin. Though also considered to be a guardian of the East, in most cases, Akshobhya is given that role. As an exceptional case, the honzon of Mount Kōya's Kongōbu Temple was changed from Akshobhya to Bhaiṣajyaguru.

Shingon (真言宗, Shingon-shū; "True Word/Mantra School") is one of the major schools of Buddhism in Japan and one of the few surviving Vajrayana lineages in East Asian Buddhism. It is a form of Japanese Esoteric Buddhism and is sometimes called "Tōmitsu" (東密 lit. "Esoteric [Buddhism] of Tō-ji"). The word shingon is the Japanese reading of the Chinese word 真言 (zhēnyán), which is the translation of the Sanskrit word mantra.

The Zhenyan lineage was founded in China (c. 7th–8th centuries) by Indian vajrācāryas (esoteric masters) like Śubhakarasiṃha, Vajrabodhi and Amoghavajra. These esoteric teachings would later flourish in Japan under the auspices of a Buddhist monk named Kūkai (空海, 774–835), who traveled to Tang China and received these esoteric transmissions from a Chinese master named Huiguo (746–805). Kūkai established his tradition at Mount Kōya (in Wakayama Prefecture), which remains the central pilgrimage center of Shingon Buddhism.