The sack of Constantinople occurred in April 1204 and marked the culmination of the Fourth Crusade. Crusader armies captured, looted, and destroyed parts of Constantinople, then the capital of the Byzantine Empire. After the capture of the city, the Latin Empire (known to the Byzantines as the Frankokratia or the Latin Occupation) was established and Baldwin of Flanders was crowned Emperor Baldwin I of Constantinople in the Hagia Sophia.

After the city's sacking, most of the Byzantine Empire's territories were divided up among the Crusaders. Byzantine aristocrats also established a number of small independent splinter states, one of them being the Empire of Nicaea, which would eventually recapture Constantinople in 1261 and proclaim the reinstatement of the Empire. However, the restored Empire never managed to reclaim its former territorial or economic strength, and eventually fell to the rising Ottoman Empire in the 1453 Siege of Constantinople.

The sack of Constantinople is a major turning point in medieval history. The Crusaders' decision to attack the world's largest Christian city was unprecedented and immediately controversial. Reports of Crusader looting and brutality scandalised and horrified the Orthodox world; relations between the Catholic and Orthodox churches were catastrophically wounded for many centuries afterwards, and would not be substantially repaired until modern times.

The Byzantine Empire was left much poorer, smaller, and ultimately less able to defend itself against the Seljuk and Ottoman conquests that followed; the actions of the Crusaders thus directly accelerated the collapse of Christendom in the east, and in the long run helped facilitate the later Ottoman Conquests of Southeastern Europe.

The Fourth Crusade (1202–1204) was a Latin Christian armed expedition called by Pope Innocent III. The stated intent of the expedition was to recapture the Muslim-controlled city of Jerusalem, by first defeating the powerful Egyptian Ayyubid Sultanate, the strongest Muslim state of the time. However, a sequence of economic and political events culminated in the Crusader army's 1204 sack of Constantinople, the capital of the Greek Christian-controlled Byzantine Empire, rather than Egypt as originally planned. This led to the partitioning of the Byzantine Empire.

In exchange for building a dedicated fleet and providing sea transport, the Republic of Venice set the condition that the Crusaders help them capture Zadar (or Zara), on the Adriatic Sea. This led in November 1202 to the siege and sack of Zara, the first attack against a Catholic city by a Catholic Crusader army. The city was then brought under Venetian control. When the Pope heard of this, he excommunicated the Crusader army. In January 1203, en route to Jerusalem, the Crusader leadership entered into an agreement with the Byzantine prince Alexios Angelos to divert the Crusade to Constantinople and restore his deposed father Isaac II Angelos as emperor. The intent of the Crusaders was then to continue to Jerusalem with promised Byzantine financial and military aid. On 23 June 1203, the main Crusader army reached Constantinople, while other contingents (perhaps a majority of all crusaders) continued to Acre.

In August 1203, following the siege of Constantinople, Alexios was crowned co-emperor. However, in January 1204 he was deposed by a popular uprising. The Crusaders were no longer able to receive their promised payments from Alexios. Following the murder of Alexios on 8 February, the Crusaders decided on the outright conquest of the city. In April 1204, they captured and plundered the city's enormous wealth. Only a handful of the Crusaders continued to the Holy Land thereafter.

The conquest of Constantinople was followed by the fragmentation of the Byzantine Empire into three states centered in Nicaea, Trebizond and Epirus. The Crusaders then founded several new Crusader states, known as Frankokratia, in former Byzantine territory, largely hinged upon the Latin Empire of Constantinople. The presence of the Latin Crusader states almost immediately led to war with the Byzantine successor states and with the Bulgarian Empire. The Nicaean Empire eventually recovered Constantinople and restored the Byzantine Empire in 1261.

The Fourth Crusade is considered to have solidified the East–West Schism. The crusade dealt an irrevocable blow to the Byzantine Empire, contributing to its decline and fall.