The first archaeological artifacts of human activity in Ruhnu, assumed to be related to seasonal seal hunting, date back to around 5000 BC. The time of arrival of the first ancient Scandinavians in Ruhnu and the beginning of a permanent Swedish-speaking settlement is not known. It probably did not precede the Northern Crusades in the beginning of the 13th century, when the indigenous peoples of all the lands surrounding the Gulf of Riga were converted to Christianity and subjugated to the Teutonic Order. The first documented record of the island of Ruhnu, and of its Swedish population, is a 1341 letter sent by the Bishop of Courland which confirmed the islanders' right to reside and manage their property in accordance with Swedish law.
Ruhnu was controlled by the Kingdom of Sweden (1621–1708, formally until 1721) and after that by Imperial Russia until World War I, when it was occupied by Imperial Germany (1915–1918). After the war, despite some local initiatives to rejoin Sweden, and territorial claims by Latvia, the islanders agreed to become part of newly independent Estonia in 1919 (possibly in exchange for Estonian military aid to Latvia against the West Russian Volunteer Army). According to a census taken in 1934, Ruhnu had a population of 282: 277 ethnic Swedes and 5 ethnic Estonians.
During World War II, Ruhnu, along with the rest of Estonia, was first occupied by the Soviet Union (1940–1941) and then by Germany (1941–1944). In November 1943, a first group of about 75 islanders relocated to Sweden. In August 1944, shortly before the beginning of the second Soviet occupation, the entire remaining population of the island, with the exception of just two families, fled by ship to Sweden.
During the period of Soviet occupation after 1944, the island was repopulated by Estonian civilians and also served as a basis of a small Soviet military garrison. The number of inhabitants never exceeded 400, and in the 1970s, after a storm hit
