The King of Thessalonica was the ruler of the Kingdom of Thessalonica, one of the crusader states founded in Greece in the aftermath of the Fourth Crusade (1202–1204). The King of Thessalonica was not an independent ruler; the Kingdom of Thessalonica was one of several vassal states created by the crusaders, subservient to the new Latin Empire of Constantinople, which had supplanted the Byzantine Empire.

The kingdom proved to be short-lived, with Thessalonica being captured by Theodore Komnenos Doukas of Epirus in 1224, just twenty years after the kingdom had been founded. The last king, Demetrius of Montferrat, escaped into exile and upon his death ceded the title to Frederick II, Holy Roman Emperor, who some years later ceded it back to Demetrius's family. Their line of titular Kings of Thessalonica ended with the marriage of Yolande of Montferrat to Byzantine emperor Andronikos II Palaiologos in 1284, at which point titular ownership of Thessalonica passed back to the Byzantine emperors.

Other lines of titular kings of Thessalonica originated in that Baldwin II, titular Latin Emperor of Constantinople, perceived Frederick II's use of the title as invalid as he was excommunicated and had been denounced as a heretic. At two points, Baldwin granted the title to nobles in Western Europe, first to Hugh IV of Burgundy and then to Philip of Sicily. Claims to the title did not cease until the 1330s, more than a century after the kingdom's fall.