Homoousian (Ancient Greek: ὁμοούσιος, from the Ancient Greek: ὁμός, homós, "same" and Ancient Greek: οὐσία, ousía, "essence, being") is a technical theological term used in discussion of the Christian understanding of God as Trinity. The Nicene Creed describes Jesus as being homooúsios with God the Father — that is, they are of the "same substance" and are equally God. This term, adopted by the First Council of Nicaea, was intended to add clarity to the relationship between Christ and God the Father within the Godhead.

The term ὁμοούσιος had been used before its adoption by the Nicene theology. The Gnostics were the first theologians to use the word "homoousios", while before the Gnostics there is no trace at all of its existence. The early church theologians were probably made aware of this concept, and thus of the doctrine of emanation, by the Gnostics. In Gnostic texts the word "homoousios" is used with these meanings: (1) identity of substance between generating and generated; (2) identity of substance between things generated of the same substance; (3) identity of substance between the partners of a syzygy. For example, Basilides, the first known Gnostic thinker to use "homoousios" in the first half of the 2nd century, speaks of a threefold sonship consubstantial with the god who is not. The Valentinian Gnostic Ptolemy claims in his letter to Flora that it is the nature of the good God to beget and bring forth only beings similar to, and consubstantial with himself. "Homoousios" was already in current use by the 2nd-century Gnostics, and through their works it became known to the orthodox heresiologists, though this Gnostic use of the term had no reference to the specific relationship between Father and Son, as is the case in the Nicene Creed.