The Teaching of Addai, usually dated c.AD 400, is a Syriac account of King Abgar of Edessa (ostensibly Abgar V who ruled from 4 BC - 7AD and again from 13-50 AD) and his contact with Jesus. Upon hearing from his servants that the Messiah was performing healings in Palestine, Abgar sends a letter to Jesus inviting him to come to Edessa to heal a certain illness that he has and to find refuge from the hostility of the Jews. Jesus receives the letter and sends word back to Abgar that His work here is finished and that he is ready to return to His heavenly Father. He informs him, however, that after He has ascended He will send one of His disciples to heal him. This disciple turns out to be Addai the Apostle, who, according to Eusebius is to be identified with Thaddaeus, one of the Seventy disciples. Addai comes to Edessa, heals Abgar, and establishes the church there. Much of the document is taken up with the teaching of Addai to the people of Edessa: hence the title, The Teaching of Addai.

The Abgar legend was very popular in the early church, especially the story about the exchange of correspondence between Jesus and Abgar. It first appears c. 325 AD in Eusebius who says that Jesus upon receiving the letter of Abgar wrote a letter in reply. In the Teaching of Addai there is no mention of Jesus writing a letter. Addai says simply that Jesus sent a reply by Hanan, Abgar's archivest. Eusebius, however, affirms that both letters were to be found in the archives of Edessa. Eusebius thereupon follows with a translation of the letters.

This text, The Teaching of Addai is one of the earliest documents that was widely circulated among the Aramaic speaking Christians. In studying the Aramaic based literature of the early Church, few documents are more important .