The Onmyōji (陰陽師, lit. Yin-Yang Masters) were court diviners in the Heian period (794–1185 AD) of Japan, and may have been active at other times. The organization they belonged to was called the Onmyōryō. Their court responsibilities ranged from simple tasks such as keeping track of the calendar, to mystical chores such as divination and protection of the capitol from evil spirits.

Their practice was based upon the Tao. Yin and Yang ruled everything. They also studied the interaction of the five oriental elements: wood, fire, metal, water, and earth. It was said that an Onmyōji could also summon and control shikigami.

One of the most famous of the Onmyōji was a man by the name of Abe no Seimei (921–1005). After his death the emperor had a temple erected at his home in Kyoto.

In anime, onmyōji frequently appear and are sometimes referred to as spiritualistic mediums. Characters include Yohmei Asakura, Yoh's grandfather in Shaman King, and Subaru Sumeragi, from X/1999 and Tokyo Babylon.