John III Doukas Vatatzes, Latinized as Ducas Vatatzes (Greek: Ιωάννης Γ΄ Δούκας Βατάτζης, Iōannēs III Doukas Vatatzēs, c. 1193, Didymoteicho – 3 November 1254, Nymphaion), 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. The Vatatzes family had first become prominent in Byzantine society during the Komnenian period and had forged early imperial connections when Theodore Vatatzes married the porphyrogennete princess Eudokia Komnene, daughter of Emperor John II Komnenos.

John Doukas Vatatzes had two older brothers. The eldest was Isaac Doukas Vatatzes (died 1261), who married and had two children: John Vatatzes (born 1215), who married to Eudokia Angelina and had two daughters: Theodora Doukaina Vatatzaina, who later married Michael VIII Palaiologos; and Maria Vatatzaina, who later married Michael Doukas Glabas Tarchaneiotes, military governor of Thrace.

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. The struggle ended with the Battle of Poimanenos in 1224, in which his opponents were defeated in spite of support from the Latin Empire of Constantinople. John III's victory led to territorial concessions by the Latin Empire in 1225, followed by John's incursion into Europe, where he seized Adrianople.

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