The siege of Constantinople of 860 was the only major military expedition of the Rus' Khaganate recorded in Byzantine and Western European sources. The casus belli was the construction of the fortress Sarkel by Byzantine engineers, restricting the Rus' trade route along the Don River in favor of the Khazars. Accounts vary, with discrepancies between contemporary and later sources, and the outcome is unknown in detail.

It is known from Byzantine sources that the Rus' caught Constantinople unprepared, while the empire was preoccupied by the ongoing Arab–Byzantine wars and was unable to respond effectively to the attack, certainly initially. After pillaging the suburbs of the Byzantine capital, the Rus' retreated for the day and continued their siege in the night after exhausting the Byzantine troops and causing disorganization. The event gave rise to a later Orthodox Christian tradition, which ascribed the deliverance of Constantinople to a miraculous intervention by the Theotokos.