Macedonius I of Constantinople, was the bishop of Constantinople during the mid-fourth century. He was an semi-Arian, and with the support of Emperor Constantius II, the Semi-Arian party was able to install him as the bishop of Constantinople. He served as bishop for two periods: from 342 to 349 and from 351 to 360.

Macedonius came to support a view that while it generally supported the Nicene-Constantinopolitan [Creed Nicene] creed rejected the Divinity of the Holy Spirit. The doctrine developed into a sect that after his death carried his name: Macedonians. The sect was also referred to as the Pneumatomachi (“Opponents of the Spirit”).

Macedonius and his followers were semi-Arian, and taught that though the Son was eternal, He was not of one essence (Latin: consubstanciales, Greek: homoousios) with the Father but of like essence (Greek: homoiousios) with the Father. They also taught that the Holy Spirit was not eternal and consubstantial with the Father and the Son but a creation of the Father and an action of the Son. Thus, the Macedonians denied that the Holy Spirit was a hypostasis, or person, of the Holy Trinity.