Joannicius the Great or Ioannikios (Greek: Όσιος Ιωαννίκιος ο Μέγας; born 762, Bithynia - November 4, 846 in Antidium) was a Byzantine Christian saint, sage, theologian and prophet.

Well known for his devoted asceticism and defense of icon veneration, Joannicius spent the majority of his life as a hermit on Mysian Olympus, near what is today Bursa, Turkey.

Icon veneration was later restored to the Byzantine Empire under the reign of Empress Theodora, a move that some devotees ascribe to Joannicius's influence and prophecies.

[1] Joannicius served in the Byzantine army in his early years before devoting his life to ascetic study and monastic contemplation.

He served there until 792, distinguishing himself by saving Emperor Constantine VI (or an officer, according to another variant) from being captured at the Battle of Marcellae.

According to the hagiographies, Joannicius spent the rest of his life as an ascetic on Uludağ, with years dedicated to solitude, study, and prayer.

Joannicius spent most of his time reciting the psalms and meditating on the life of Jesus, but the hagiographies also ascribe some miracles and prophecies to him.