Ioannes III

Ioannes III (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