Homoiousios (Greek: ὁμοιούσιος from ὅμοιος, hómoios, "similar" and οὐσία, ousía, "essence, being") is a heretical Christian theological term, coined in the 4th century to identify a distinct group of Christian theologians who held the belief that God the Son was of a similar, but not identical, essence (or substance) with God the Father.

During the period of the development of Christian doctrine and refinement of Christian theological language which ran from AD 360 to 380, the controversy between Arianism and what would eventually come to be defined as catholic orthodoxy provoked an enormous burgeoning of new movements, sects and doctrines which came into existence in the attempt to stabilize and consolidate a unique and universal position on complex and subtle theological questions. One of the central questions concerned the nature of God and the fundamental character of his relationship with his Son Jesus Christ as the preexistent Logos. This controversy was called the "trinitarian controversy" because it involved solving the riddle of how it was possible that God the Father, His Son Jesus the Word, and the Holy Spirit could be one God. The dominant position among Christian theologians at this point in history was the doctrine of homoousianism, articulated and fiercely defended by Athanasius of Alexandria, according to which Father and Son were identical in essence, divine identity, attributes and energies, and that any deviations from this orthodoxy were to be considered heretical departures from apostolic faith and worship. The Homoians, however, had a powerful ally on their side in the person of Emperor Constantius II.

It is often claimed that Homoiousianism arose as an attempt to reconcile two opposing teachings, namely, Homoousianism and Homoianism:

It is then proposed that Homoiousian (similar substance) theology was an attempt to reconcile the Homoousian (same substance) theology with the Homoian notion of similarity.

However, Homoiousianism was “most prominently associated with … Basil of Ancyra” and "the term homoiousios plays no role in Basil's surviving texts," implying that such a compromise was not the purpose. More recently, Lewis Ayres proposed that Homoiousianism was not merely a compromise but "a significant and persistent strand in earlier eastern theology."

There are indications that this theology was a restatement or development of the theology of Eusebius of Caesarea, as stated in the letter he wrote to his home church after the Nicene Council, to explain why he accepted that Creed:

Eusebius was “universally acknowledged to be the most scholarly bishop of his day.”

Both Lewis Ayres and R.P.C. Hanson stated that the formulation of Homoiousian theology in 358 by a council of bishops called by Basil of Ancyra was a response to, what Ayres calls, "the emerging shape of Heterousian theology" in the form of the creed of “Sirmium 357,” which was based on the teachings of Aetius. Hanson refers to this as "Neo-Arianism" and as “a new and radical theology” that appears for the first time in the extant ancient records in the form of the “Second Creed of Sirmium of 357,” afterward approved by a larger synod at Antioch (probably in AD 358). 'Neo-Arianism' may be an appropriate name because it was “a development" of Arius' theology.