John III Doukas Vatatzes, Latinized as Ducas Vatatzes (Greek: Ιωάννης Γ΄ Δούκας Βατάτζης), was Emperor of Nicaea from 1222 to 1254. He was succeeded by his son, known as Theodore II Laskaris.
John Doukas Vatatzes, born in about 1192 in Didymoteicho, was probably the son of the general Basileios Vatatzes, Duke of Thrace, who died in 1193, and his wife, an unnamed daughter of Isaakios Angelos and cousin of the Emperors Isaac II Angelos and Alexios III Angelos. A successful soldier from a military family, John was chosen in about 1216 by Emperor Theodore I Laskaris as the second husband for his daughter Irene Laskarina and as heir to the throne, following the death of her first husband, Andronikos Palaiologos. This arrangement excluded members of the Laskarid family from the succession, and when John III Doukas Vatatzes became emperor in mid-December 1221, following Theodore I's death in November, he had to suppress opposition to his rule.
John II died in Nymphaion in 1254, and was buried in the monastery of Sosandra, which he had founded, in the region of Magensia.