The sack of Thessalonica in 1185 by Normans of the Kingdom of Sicily was one of the worst disasters to befall the Byzantine Empire in the 12th century.
David Komnenos, the governor, had failed to prepare for the siege and banned defenders from disrupting Norman siege works, hindering the city's defense.The Byzantine relief armies failed to coordinate their efforts, and only two forces, under Theodore Choumnos and John Maurozomes, actually came to the city's aid.
The conquest degenerated quickly into a full-scale massacre of the city's inhabitants, with estimates suggesting that around 7,000 to 8,000 corpses were found afterwards.
The Normans occupied Thessalonica until mid-November, when, following their defeat at the Battle of Demetritzes, they evacuated it.
It also directly led to the deposition and execution of the unpopular Emperor Andronikos and the rise to the throne of Isaac II Angelos.