Byzantine emperor of Nicaea who—though not as capable as his grandfather or his father, Theodore I and John III Vatatzes, respectively—was an able ruler, a good soldier, and a man of letters; he succeeded in holding together the prosperous state east of Constantinople bequeathed to him by his father.

Theodore II, who had already been crowned as coemperor, became sole ruler on the death of his father in November 1254. At the outset of his reign, he renewed the alliance with the Seljuq sultanate of Rūm (in modern Turkey). Early in 1255, however, the Bulgar tsar Michael II Asen invaded Thrace and Macedonia. After two victorious campaigns against the Bulgars (1255–56), Theodore compelled them to sign a treaty (May 1256).

In October 1256 Theodore's daughter Maria married Nicephorus, son of Michael, the despot of Epirus. As a condition of the marriage, however, Theodore demanded the towns of Dyrrachium (now Durrës, Albania) and Servia (in Greece). Michael was enraged at this demand, and war broke out; it was still being waged at the time of Theodore's death.