The word Rus' or Rus referred initially to a group of Scandinavian Vikings, also known as Varangians, who founded the medieval state of Kievan Rus' in Eastern Europe in the 10th century. The term gradually acquired the meaning of the aforementioned dynastic polity itself, and also the geographic region of its heartlands Kiev, Pereiaslavl' and Chernihiv. Russia is a Hellenized rendering of the same word, and Ruthenia is its Latinized form.

Following the decline of Kievan Rus' in the 12th century, its territory fragmented into multiple polities. The northeastern principality of Vladimir-Suzdal played a crucial role in the eventual rise of the Grand Duchy of Moscow, which, by the 14th to 16th centuries, had consolidated power over most of northeastern Rus'. The name Russia began to appear in official documents during this time, alongside the older term Rus'. By the 15th century, Muscovite rulers adopted the title "Grand Prince of all Rus'," signaling their claim over the legacy of Kievan Rus'. The term Russia gradually replaced Rus', and by the 16th century, under Ivan IV, the state officially became the Tsardom of Russia. Despite this, the term Muscovy persisted in Europe, especially in Latin Catholic regions, but Russia was increasingly recognized across Northern Europe and the courts of the Holy Roman Empire.

The name Ruthenia originated as a Latinized form of Rus' and was commonly used in Western European documents to refer to the eastern Slavic lands during medieval times. Over time, the name became more localized, especially after the 19th century, to refer to Carpathian Ruthenia—a region in the northeastern Carpathian Mountains inhabited by Slavs with a Rusyn identity.

Initially the ecclesiastical title "Metropolitan of Kiev and all Rus'" was used for the head of the church based in Kiev until the metropolitan see moved to Moscow in the 14th century, where it became "Patriarch of Moscow and all Rus'" with the establishment of the Russian Orthodox Church. In contrast, the southwestern regions of former Kievan Rus' adopted the title "Metropolitan of Kiev, Galicia and all Ruthenia", while modern Ukrainian Orthodox churches have shifted to using titles reflecting "Ukraine" instead of "Rus'."

The most common theory about the origins of Russians is the Germanic version. The name Rus', like the Proto-Finnic name for Sweden (*roocci), supposed to be descended from an Old Norse term for "the men who row" (rods-) as rowing was the main method of navigating the rivers of Eastern Europe, and that it could be linked to the Swedish coastal area of Roslagen or Roden, as it was known in earlier times. The name Rus' would then have the same origin as the Finnish, Estonian, Võro and Northern Sami names for Sweden: Ruotsi, Rootsi, Roodsi and Ruoŧŧa. The local Finnic and Permic peoples in northern Russia proper use the same (Rus'-related) name both for Sweden and Russia (depending on the language): thus the Veps name for Sweden and Swedish is Ročinma / Ročin, while in the Komi language spoken further east the etymologically corresponding term Roćmu / Roć means already Russia and Russian instead. The Finnish scholar Tor Karsten has pointed out that the territory of present-day Uppland, Södermanland and Östergötland in ancient times was known as Roðer or roðin. Thomsen accordingly has suggested that Roðer probably derived from roðsmenn or roðskarlar, meaning seafarers or rowers. Ivar Aasen, the Norwegian philologist and lexicographer, noted proto-Germanic root variants Rossfolk, Rosskar, Rossmann.

George Vernadsky theorized about the association of Rus and Alans. He claimed that Ruxs in Alanic means "radiant light", thus the ethnonym Roxolani could be understood as "bright Alans". He theorized that the name Roxolani a combination of two separate tribal names: the Rus and the Alans.

According to the Oxford English Dictionary, the English name Russia first appeared in the 14th century, borrowed from Medieval Latin Russia, which was in turn a rendition of the Byzantine Greek name for Rus', Ρωσία (Rosía). The word Ruthenia originated as a Latin designation of the region its people called Rus'. Rusia or Ruthenia appears in the 1520 Latin treatise Mores, leges et ritus omnium gentium, per Ioannem Boëmum, Aubanum, Teutonicum ex multis clarissimis rerum scriptoribus collecti by Johann Boemus. In the chapter De Rusia sive Ruthenia, et recentibus Rusianorum moribus ("About Rus', or Ruthenia, and modern customs of the Rus'"), Boemus tells of a country extending from the Baltic Sea to the Caspian Sea and from the Don River to the northern ocean. It is a source of beeswax, its forests harbor many animals with valuable fur, and the capital city Moscow (Moscovia), named after the Moskva River (Moscum amnem), is 14 miles in circumference. Danish diplomat Jacob Ulfeldt, who traveled to Russia in 1578 to meet with Tsar Ivan IV, titled his posthumously (1608) published memoir Hodoeporicon Ruthenicum ("Voyage to Ruthenia").

In Old East Slavic literature, the East Slavs refer to themselves as "[muzhi] ruskie" ("Rus' men") or, rarely, "rusichi." The East Slavs are thought to have adopted this name from the Varangian elite,^{[citation needed]} which was first mentioned in the 830s in the Annales Bertiniani. The Annales recount that Louis the Pious's court at Ingelheim am Rhein in 839 (the same year as the first appearance of Varangians in Constantinople), was visited by a delegation from the Byzantine emperor. The delegates included two men who called themselves "Rhos" ("Rhos vocari dicebant"). Louis inquired about their origins and learned that they were Swedes. Fearing that they were spies for their brothers the Danes, he jailed them. They were also mentioned in the 860s by Byzantine Patriarch Photius under the name "Rhos."^{[citation needed]}