Agathangelos (in Old Armenian: Ագաթանգեղոս Agatʿangełos, in Greek Ἀγαθάγγελος "bearer of good news" or angel, c. 5th century AD^[1]) is the pseudonym of the author of a life of the first apostle of Armenia, Gregory the Illuminator, who died about 332.^[2] The history attributed to Agathangelos is the main source for the Christianization of Armenia in the early 4th century.

The "standard" version of Agathangelos' history accepted in the Armenian tradition dates to the second half of the 5th century. This version was soon translated into Greek; on the basis of this Greek translation, a translation into Arabic was made, as well as many secondary Greek, Latin and Ethiopic versions. Another, earlier Armenian version of the history, now lost, was the basis for two Greek, two Arabic, and a Karshuni translation.^[3]

He claims to be a secretary of Tiridates III, King of Armenia in the early 4th century. Some researchers assume that the life might have been written after the 5th century.^[2] It purports to exhibit the deeds and discourses of Gregory, and has reached us in Armenian, Greek, Georgian, Syriac, Ethiopic, Latin and Arabic.^[4] The text of this history has been altered by some modern researches, but it has always been in high favor with the Armenians and the early medieval scholars who translated it into different languages.^[5] Von Gutschmid maintains that the unknown author made use of a genuine life of St. Gregory and of the martyrdom of Saint Rhipsime and her companions. Historical facts are intermingled in this life with legendary or uncertain additions, and the whole is woven into a certain unity by the narrator, who may have assumed his significant name from his quality of narrator of "the good news" of Armenia's conversion.^[6] It has been translated into several languages, and Greek and Latin translations are found in the Acta Sanctorum Bollandistarum, tome viii.^[5]

According to Agathangelos, he was tasked by Tiridates III to write about his father Khosrov II of Armenia and his reign period. Until the 19th century, based on this fact, scholars believe that Agathangelos lived in the 4th century. Later there attempts were made by researchers to demonstrate that Agathangelos lived and worked in the 5th century and was not able to be Tiridates III's secretary.^{[citation needed]}. The opinion among many scholars however prevails that the systemic approach of the Armenian historiography allows calculating the approximate dates of the most authors. For instance, Ghazar Parpetsi, whose biographical data are known (second half of Vth century – beginning of VIth) century calls him the author of the “Third Book”. He calls P'awstos Buzand born before him the author of the “Second Book” and Agathangelos, the author of the “First Book”. Buzand has the same approach. Also, in his book each author focuses in greater details on the events happening during or close to his own live: Parpetsi's focus was the liberation wars led by Mamikonyans (450 - 484), Buzand describes in more details events of the second half of the IV century, and Agathangelos provides details of the adoption of Christianity and events in the first half of the IV century.^[7]