Theodore II Doukas Laskaris or Ducas Lascaris (Greek: Θεόδωρος Δούκας Λάσκαρις, romanized: Theodōros Doúkās Láskaris; November 1221/1222 – 16 August 1258) was Emperor of Nicaea from 1254 to 1258. He was the only child of Emperor John III Doukas Vatatzes and Empress Irene Laskarina. His mother was the eldest daughter of Theodore I Laskaris, who had established the Empire of Nicaea as a successor state to the Byzantine Empire in Asia Minor after the crusaders captured the Byzantine capital, Constantinople, during the Fourth Crusade in 1204. Theodore received an excellent education from two renowned scholars, Nikephoros Blemmydes and George Akropolites. He made friends with young intellectuals, especially with a page of low birth, George Mouzalon. Theodore began to write treatises on theological, historical and philosophical themes in his youth.
Emperor John III arranged for Theodore to marry Elena Asenina in 1235, to forge an alliance with her father, Ivan Asen II, Emperor of Bulgaria, against the Latin Empire of Constantinople. According to Theodore himself, their marriage was happy, and they had five or six children. From 1241, Theodore acted as his father's lieutenant in Asia Minor during his frequent military campaigns in the Balkan Peninsula. From around 1242, he was his father's co-ruler, but was not crowned as co-emperor. During this period, his relationship with some prominent aristocrats, particularly Theodore Philes and Michael Palaiologos, grew tense.