first emperor of Nicaea, which was recognized as the Byzantine government-in-exile and as the legitimate successor of the Byzantine Empire during the Crusaders' occupation of Constantinople (Istanbul).

He was a son-in-law and heir of the Byzantine emperor Alexius III Angelus. After the Byzantine capital fell to the Fourth Crusade in 1204, Theodore gathered a band of refugees, first at Brusa and then at Nicaea, across the Bosporus in Asia Minor, and formed a new Byzantine state. In 1208 he assumed the title of emperor and defended his infant empire not only against the Crusaders but also against David Comnenus, a rival Greek emperor in Trebizond to the east on the Black Sea, and against the Seljuq Turks. When the Seljuq sultan of Rūm, Kay-Khusraw, who had given asylum to the emperor Alexius, failed to persuade Theodore to abdicate, he invaded Theodore's territory in the spring of 1211. Theodore, however, defeated and killed Kay-Khusraw in battle and also captured and imprisoned Alexius.

After a period of warfare with Henry (Henry of Hainault) of Flanders, Latin emperor of Constantinople, Theodore signed a treaty (c. 1214) defining the frontiers between the Greek empire of Nicaea and the Latin empire of Constantinople. Theodore then annexed much of the territory of Trebizond. After Henry's death (1216), Theodore strengthened his ties to the Latin empire by taking as his third wife Maria, daughter of the empress Yolande, and also by proposing (1219) that Greek and Latin clergy meet in Nicaea to consider the reunion of the two churches.

In August 1219 Theodore made a lucrative commercial agreement with the Venetians in Constantinople. In 1222, shortly before his death, he negotiated a settlement with Yolande's son and successor as Latin emperor, Robert of Courtenay, to whom he betrothed his daughter Eudocia. On his death Theodore was succeeded as emperor of Nicaea by his son-in-law John III Vatatzes.