Theodore I (Theodore Lascaris), d. 1222, Byzantine emperor of Nicaea (1204-22), son-in-law of the Byzantine emperor Alexius III. He escaped from Constantinople after it was captured (1204) by the Latins of the Fourth Crusade and founded a Byzantine state at Nicaea (see Nicaea, empire of). Uniting nearly all of W Asia Minor except the Turkish sultanate of Iconium, he kept his state intact against Henry of Flanders, Latin emperor of Constantinople, and against the Seljuk Turks. He was succeeded by his son-in-law, John III.