The Battle of al-Qadisiyyah (Arabic: مَعْرَكَة ٱلْقَادِسِيَّة Maʿrakat al-Qādisīyah; Persian: نبرد قادسیه Nâbārd-e Qâdisiyeh) took place between the armies of the Rashidun Caliphate and the Sasanian Empire in November 636. It resulted in a decisive victory for the Rashidun army and is considered to be one of the most significant engagements of the Muslim conquest of Persia, and thereby of the early Muslim conquests as a whole. After losing al-Qadisiyyah, the Sasanian army was left unable to defend against the Rashidun siege of Ctesiphon, thus being forced to retreat from all of Mesopotamia. This development enabled further Rashidun offensives into the Persian mainland and culminated in the Sasanian Empire's annexation by 651.
It is widely believed that the Rashidun army's advance on al-Qadisiyyah began on 16 November 636 and continued for the next three days. During the battle, Sasanian troops were thrown into disarray following the death of their general Rostam Farrokhzad under uncertain circumstances. The ensuing collapse of their positions marked a cataclysmic defeat that led to the Rashidun army's seizure of Ctesiphon, which had served as the Persian capital city since the Parthian period.