Ammonius of Alexandria was a Christian theologian who lived during the 3rd century CE. He was active in the city of Alexandria, Egypt, a major center where Greek philosophical learning and early Christian thought interacted. Details about his life are sparse, and he is often confused with other Alexandrian intellectuals named Ammonius, particularly non-Christian philosophers.

He is known for a single work, cited by the 4th-century historian Eusebius, titled On the Harmony of Moses and Jesus. This treatise is now lost, and its arguments are known only from Eusebius's brief reference. According to modern scholars, the work was part of a broader effort within Alexandrian Christianity to demonstrate the philosophical consistency of Christian doctrine and to reconcile the Hebrew scriptures (associated with Moses) with the teachings of Jesus.

His historical significance lies in his place within the early Christian intellectual tradition. He represents the ongoing project of theologians in his time and city to engage with philosophical frameworks and present Christian teachings as a unified system. While his direct influence was limited due to the loss of his writing, his mention by Eusebius confirms his role as a 3rd-century author contributing to theological debate and scriptural interpretation.