Theodore Lascaris (d. 1222), emperor of Nicaea, was born of a noble Byzantine family.

He became the son-in-law of the Byzantine Emperor Alexius III and distinguished himself during the sieges of Constantinople by the Latins (1203-1204). After the capture of the city he gathered a band of fugitives in Bithynia and established himself in the town of Nicaea, which became the chief rallying-point for his countrymen.

Relieved of the danger of invasion by a Latin force which had defeated him in 1204 but was recalled to Europe by a Bulgarian invasion, he set to work to form a new Byzantine state in Asia Minor, and in 1206 assumed the title of emperor.

During the next years Theodore was beset by enemies surrounding his fledgling state. He maintained himself stubbornly in defensive campaigns against the Latin emperor Henry of Flanders, defeated his rival Alexius I , emperor of Trebizond, and carried out a successful counter-attack upon Kay Khusrau I , the sultan of Rüm (also called the sultan of Iconium or Konya), who had been instigated to war by the deposed Alexius III.

Theodore's crowning victory was gained in 1210, when in a battle near Pisidian Antioch he captured Alexius and wrested the town itself from the Turks.

At the end of his reign he ruled over a territory roughly conterminous with the old Roman provinces of Asia and Bithynia. Though there is no proof of higher qualities of statesmanship in him, by his courage and military skill he enabled the Byzantine nation not merely to survive, but ultimately to beat back the Latin invasion.