The Siege of Baghdad was a siege that took place in Baghdad in 1258, lasting for 13 days from January 29, 1258 until February 10, 1258. The siege, laid by Ilkhanate Mongol forces and allied troops, involved the investment, capture, and sack of Baghdad, which was the capital of the Abbasid Caliphate at that time. The Mongols were under the command of Hulagu Khan, brother of the khagan Möngke Khan, who had intended to further extend his rule into Mesopotamia but not to directly overthrow the Caliphate. Möngke, however, had instructed Hulagu to attack Baghdad if the Caliph Al-Musta'sim refused Mongol demands for his continued submission to the khagan and the payment of tribute in the form of military support for Mongol forces in Persia.

Hulagu began his campaign in Persia against the strongholds of Nizari Ismailis, who lost their stronghold of Alamut. He then marched on Baghdad, demanding that Al-Musta'sim accede to the terms imposed by Möngke on the Abbasids. Although the Abbasids had failed to prepare for the invasion, the Caliph believed that Baghdad could not fall to invading forces and refused to surrender. Hulagu subsequently besieged the city, which surrendered after 12 days.

During the next week, the Mongols sacked Baghdad, committing numerous atrocities there is debate among historians about the level of destruction of library books and the Abbasids' vast libraries. The Mongols executed Al-Musta'sim and massacred many residents of the city, which was left greatly depopulated. The siege is considered to mark the end of the Islamic Golden Age, during which the caliphs had extended their rule from the Iberian Peninsula to Sindh, and which was also marked by many cultural achievements in diverse fields.