The Battle of Pelagonia or Battle of Kastoria took place in early summer or autumn 1259, between the Empire of Nicaea and an anti-Nicaean alliance comprising Despotate of Epirus, Kingdom of Sicily and the Principality of Achaea. It was a decisive event in the history of the Eastern Mediterranean, ensuring the eventual reconquest of Constantinople and the end of the Latin Empire in 1261.

The Empire of Nicaea or the Nicene Empire was the largest of the three Byzantine Greek rump states founded by the aristocracy of the Byzantine Empire that fled when Constantinople was occupied by Western European and Venetian armed forces during the Fourth Crusade, a military event known as the Sack of Constantinople. Like the other Byzantine rump states that formed due to the 1204 fracturing of the empire, such as the Empire of Trebizond and the Despotate of Epirus, 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.