The Russo-Circassian War (Adyghe: Урыс-адыгэ зауэ, romanized: Wurıs-adığə zawə; Russian: Русско-черкесская война; 1763–1864; also known as the Russian invasion of Circassia) was the invasion of Circassia by Russia, starting in July 17, 1763 (O.S) with the Russian Empire assuming authority in Circassia, followed by the Circassian refusal, and ending 101 years later with the last army of Circassia defeated on 21 May 1864 (O.S), making it exhausting and casualty-heavy for both sides. The Circassians fought the Russians longer than all the other peoples of the Caucasus, and the Russo-Circassian War was the longest war both Russia and Circassia have ever fought.

During and after the war, the Russian Empire employed a genocidal strategy of systematically massacring civilians which resulted in the Circassian genocide where up to 2,000,000 Circassians (85-97% of the total population) were either killed or expelled to the Ottoman Empire (especially to modern-day Turkey; see Circassians in Turkey), creating the Circassian diaspora. While the war was initially an isolated conflict, Russian expansion through the entire region soon drew a number of other nations in the Caucasus into the conflict. As such, the war is often considered the western half of the Caucasus War.

During the war, the Russian Empire did not recognize Circassia as an independent region, and as a result, it considered Circassia Russian land which was under rebel occupation, despite the fact that the region was not and had never been under Russian control. Russian generals did not refer to the Circassians by their ethnic name, instead, they called the Circassians "mountaineers", “bandits", and "mountain scum". The war has been subjected to historical revisionism and it has also garnered controversy due to the fact that later Russian sources mostly ignored or belittled the conflict, and Russian state media and officials have gone as far as to claim that the conflict "never happened” and they have also claimed that Circassia "voluntarily joined Russia in the 16th century".