Semi-Arianism was a position regarding the relationship between God the Father and the Son of God, adopted by some 4th-century Christians. Though the doctrine modified the teachings of Arianism, it still rejected the doctrine that Father, Son, and Holy Spirit are co-eternal, and of the same substance, or consubstantial, and was therefore considered to be heretical by many contemporary Christians.

Arius held that the Father, Son, and Holy Spirit were three separate essences or substances (ousia) and that the Son and Spirit derived their divinity from the Father, were created, and were inferior to the Godhead of the Father. Semi-Arians asserted that the Son was "of a similar substance" (homoiousios) as the Father but not "of the same substance" (homoousios).