The Kingdom of Thessalonica was a short-lived Crusader State founded after the Fourth Crusade.

Boniface of Montferrat, the leader of the crusade, was expected by both the Crusaders and Byzantines to become the new emperor after the conquest of Constantinople in 1204. However, the Venetians felt Boniface was too closely tied to the Byzantine Empire, as his brother Conrad had married into the Byzantine royal family. The Venetians wanted an emperor whom they could control more easily, and elected Baldwin of Flanders emperor of the new Latin Empire.

Boniface reluctantly accepted this, and set out to conquer Thessalonica, the second-largest Byzantine city after Constantinople. At first he had to compete with Emperor Baldwin, who also wanted the city, but Boniface won this dispute by handing over his territory on Crete to the Venetians. He then went on to capture the city later in 1204 and set up a kingdom there, subordinate to Baldwin, although the title of "king" was never officially used.

The kingdom occupied land along the Aegean coast of Thrace, Thessaly, and Macedonia, but the interior borders were undefined as the kingdom was constantly at war with the Bulgarians, who wanted to capture the remnants of the Byzantine Empire for themselves, and the Despotate of Epirus, one of the Byzantine states-in-exile trying to reconquer Constantinople. The kingdom also faced attacks from the deposed Byzantine emperor Alexius III, who had fled to Corinth, although he was quickly defeated. After this victory Boniface captured the island of Euboea and helped some other Crusaders establish the Duchy of Athens and the Principality of Achaea, which became vassal states of Thessalonica.

Boniface's rule lasted less than two years before he was ambushed by Kaloyan of Bulgaria and killed on September 4, 1207. The kingdom passed to Boniface's son Demetrius, who was still a child, so actual power was held by various minor nobles. These nobles immediately rebelled against the Latin Empire, but they were crushed by Emperor Henry of Flanders in 1209. Henry's brother Eustache then became regent for Demetrius. Taking advantage of this situation, Michael I of Epirus, a former ally of Boniface, attacked the kingdom in 1210, as did the Bulgarians. Henry of Flanders eventually defeated both. Michael's brother Theodore continued the assault on the kingdom after Michael's death in 1215. Over the next nine years Theodore gradually conquered all of Thessalonica except the city itself, as the Latin Empire could spare no army to defend it while they were busy fighting the more powerful Byzantine Empire of Nicaea. In 1224, just as Demetrius had become old enough to take power for himself, Theodore finally captured Thessalonica and the kingdom became part of the Despotate of Epirus.

The kingdom was claimed by titular kings until 1316 and was for some time nominally inherited by the Dukes of Burgundy.

Kings of Thessalonica

• Boniface of Montferrat (1204-1207)
• Demetrius of Montferrat (1207-1224)
  • ? (regent, 1207-1209)
  • Eustache of Flanders (regent, 1209-1216)
  • ? (regent, 1216-1224)

Titular Kings of Thessalonica

• Demetrius of Montferrat (1225-1230)
• Frederick II, Holy Roman Emperor (1230-1239)
• Boniface II of Montferrat (1239-1253)
• William V of Montferrat (1253-1284)
• Hugh IV of Burgundy (1266-1271) (rival claimant)
• Robert II of Burgundy (1273-1305) (rival claimant until 1284)
• Hugh V of Burgundy (1305-1313)
• Louis of Burgundy (1313-1316) (kingdom is not claimed after 1316)

See also: Montferrat