A canon (Greek: κανών, romanized: kanōn) is a structured hymn used in a number of Eastern Orthodox services. It consists of nine odes, based on the Biblical canticles. Most of these are found in the Old Testament, but the final ode is taken from the Magnificat and Song of Zechariah from the New Testament.

The roots of this type of hymn go as far back as the 5th century, reaching maturity in the Greek language through the work of St. Andrew of Crete, whose penitential Great Canon is still used on certain occasions during Great Lent. It was further developed in the 8th century by Sts. John of Damascus and Cosmas of Jerusalem, and in the 9th century by Sts. Joseph the Hymnographer and Theophanes the Branded.

Over time the canon came to replace the kontakion, a vestigial form of which is still used on several occasions and which has been incorporated into the performance of the canon. Each canon develops a specific theme, such as repentance or honouring a particular saint. Sometimes more than one canon can be chanted together, as frequently happens at Matins.