Constantine III may refer to:

• Constantine III (Western Roman emperor), self-proclaimed western Roman Emperor 407–411
• Heraclius Constantine, Byzantine Emperor in 641
• Constans II, Byzantine emperor 641–668, sometimes referred to under this name
• Constantine III of Abkhazia, king of Abkhazia in 898/99–916/17
• Constantine III of Scotland, king of Scotland 995–997
• Patriarch Constantine III of Constantinople (ruled 1059–1063)
• Constantine III of Armenia (ruled 1344–1362)
• Constantine III of Cilicia, Catholicos of the Holy See of Cilicia in 1323–1326
• Constantine III, Prince of Mukhrani (1696–1756), Georgian prince