The Empire of Nicaea or the Nicene Empire is the conventional historiographic name for the largest of the three Byzantine Greek rump states founded by the aristocracy of the Byzantine/Roman Empire that fled after Constantinople was occupied by Western European and Venetian armed forces during the Fourth Crusade, a military event known as the Sack of Constantinople. Like other Byzantine rump states that formed after the 1204 fracturing of the empire, such as the Empire of Trebizond and the Empire of Thessalonica, it was a continuation of the eastern half of the Roman Empire that survived well into the medieval period. A fourth state, known in historiography as the Latin Empire, was established by an army of Crusaders and the Republic of Venice after the capture of Constantinople and the surrounding environs.

Founded by the Laskaris family, it lasted from 1204 to 1261, when the Nicenes restored the Byzantine Empire in Constantinople after its recapture. Thus, the Nicene Empire is seen to be a direct continuation of the Byzantine Empire, as it had fully assumed the traditional titles and government of the Byzantines in 1205.

The Despotate of Epirus contested the claim in 1224 and became the Empire of Thessalonica, but was forced to renounce their claim by the Nicenes in 1242. The Empire of Trebizond, which declared its independence a few weeks before the Sack of Constantinople in 1204, eventually rescinded all claims to being a continuation of the Byzantine regime in the Treaty of 1282.