Greek bishop of Constantinople (Istanbul) and a leading moderate Arian theologian in the 4th-century Trinitarian controversy. His teaching concerning the Son, or Logos (Greek: “the Word”), oscillated between attributing to him an “identity of essence” (Greek: homoousios) and “perfect similarity” with the divinity of the Father, or Godhead. After Macedonius' death about 362, a heretical Christian sect that rejected the divinity of the Holy Spirit arose; because of the similarity of their teaching to Macedonius' doctrine of the Son, they were called Macedonians (see Macedonianism).

About 339 Macedonius usurped the episcopal throne of Constantinople from the orthodox incumbent with the support of the Arian faction, a heretical group that denied the absolute divinity of the Son. Except for the conservative, or orthodox, ascendancy (346–351), he held office until 360. Although he maintained an ambiguous theological stance, he repressed the orthodox Nicene element in Constantinople. Owing to his semi-Arian orientation or to political differences, he lost favour with the Roman emperor Constantius II (reigned 337–361) and, at a local church council in 360, was deposed and exiled.