Homoiousianism (from the Greek: ὁμοιούσιος from ὅμοιος, hómoios, "similar" and οὐσία, ousía, "essence, being") was a 4th century AD movement which arose in the early period of the Christian religion out of a wing of Arianism. It was an attempt to reconcile the seemingly irreconcilable views of the pro-Nicene homoousians, who believed that God the Father and Jesus his son were identical (ὁμός, homós) in substance, with the "neo-Arian" position that God the Father is "incomparable" and therefore the Son of God can not be described in any sense as "equal in substance or attributes" but only "like" (ὅμοιος, hómoios) the Father in some suborbinate sense of the term.

Homoiousia is the theological doctrine that Jesus the Son of God and God the Father are of similar (ὁμοιο- homoio- or homeo-) but not the same substance, a position held by the Semi-Arians in the 4th century. It contrasts with the homoousia of orthodox Trinitarianism and the heteroousia of Arianism.