Ioannes III (Greek: Ἰωάννης Γ΄, Iōannēs G΄) may refer to:
- Patriarch John III of Constantinople (died in 577)
- John III Doukas Vatatzes (c. 1192–1254), Byzantine Emperor
- John III Megas Komnenos (c. 1321–1362), Emperor of Trebizond
See also
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Ioannes III (Greek: Ἰωάννης Γ΄, Iōannēs G΄) may refer to: