The Prince of Achaea was the ruler of the Principality of Achaea, one of the crusader states founded in Greece in the aftermath of the Fourth Crusade (1202–1204). The principality witnessed various overlords during its more than two centuries of existence, initially, Achaea was a vassal state of the Kingdom of Thessalonica under Boniface I of house Montferrat, then of the Latin Empire of Constantinople under the houses of Flanders-Courtenay, which had supplanted the Byzantine Empire, and later of the Angevin Kingdom of Naples. During the Angevin period, the princes were often absent, represented in the Principality by their baillis, who governed in their name. After 1404 the principality became sovereign as the Genoese Centurione II Zaccaria bought from the Neapolitan crown the princely rights.

The Principality of Achaea or Principality of Morea was one of the vassal states of the Latin Empire, which replaced the Byzantine Empire after the capture of Constantinople during the Fourth Crusade. It became a vassal of the Kingdom of Thessalonica, along with the Duchy of Athens, until Thessalonica was captured by Theodore, the despot of Epirus, in 1224. After this, Achaea became for a while the dominant power in Greece.