Kievan Rus' (Old East Slavic: Роусь, romanized: Rusĭ, or роусьскаѧ землѧ, romanized: rusĭskaę zemlę, "Rus' land") was a loose federation in Eastern Europe and Northern Europe from the late 9th to the mid-13th century. Encompassing a variety of polities and peoples, including East Slavic, Baltic, and Finnic, it was ruled by the Rurik dynasty, founded by the Varangian prince Rurik. The modern nations of Belarus, Russia, and Ukraine all claim Kievan Rus' as their cultural ancestors, with Belarus and Russia deriving their names from it. At its greatest extent in the mid-11th century, Kievan Rus' stretched from the White Sea in the north to the Black Sea in the south and from the headwaters of the Vistula in the west to the Taman Peninsula in the east, uniting the majority of East Slavic tribes.

Rurik of Ladoga (also Ryurik or Rorik; Old East Slavic: Рюрикъ Rjurikŭ, from Old Norse Hrøríkʀ; Belarusian: Рурык; Russian, Ukrainian: Рюрик; c. 824–879), according to the 12th-century Primary Chronicle, was a Varangian chieftain of the Rus' who in the year 862 was invited to reign in Ladoga or Novgorod. This legendary figure was considered by later rulers to be the founder of the Rurik dynasty, which ruled Ladoga, then early Novgorod, and subsequently the Kievan Rus' and its successor states, including the Kingdom of Ruthenia, the Principality of Tver, Grand Duchy of Vladimir, the Grand Duchy of Moscow, the Novgorod Republic and the Tsardom of Russia, until the 17th century.

According to the Primary Chronicle, the first ruler to start uniting East Slavic lands into what would become Kievan Rus' was Prince Oleg (879–912). He extended his control from Novgorod south along the Dnieper river valley to protect trade from Khazar incursions from the east, and moved his capital to the more strategic Kiev. Sviatoslav I (943–972) achieved the first major expansion of Kievan Rus' territorial control, fighting a war of conquest against the Khazars. Vladimir the Great (980–1015) introduced Christianity with his own baptism and, by decree, extended it to all inhabitants of Kiev and beyond. Kievan Rus' reached its greatest extent under Yaroslav the Wise (1019–1054); his sons assembled and issued its first written legal code, the Russkaya Pravda, shortly after his death.

The state began to decline in the late 11th century, gradually disintegrating into various rival regional powers throughout the 12th century. It was further weakened by external factors, such as the decline of the Byzantine Empire, its major economic partner, and the accompanying diminution of trade routes through its territory. It finally fell to the Mongol invasion of the 1240s, though the Rurik dynasty would continue to rule parts of Rus' until the 14th century in the Kingdom of Galicia–Volhynia and until the 16th century after the establishment of the Tsardom of Russia.

Prior to the emergence of Kievan Rus' in the 9th century AD, the lands between the Baltic Sea and Black Sea were primarily populated by eastern Slavic tribes. In the northern region around Novgorod were the Ilmen Slavs and neighboring Krivichi, who occupied territories surrounding the headwaters of the West Dvina, Dnieper, and Volga Rivers. To their north, in the Ladoga and Karelia regions, were the Finnic Chud tribe. In the south, in the area around Kiev, were the Poliane, a group of Slavicized tribes with Iranian origins, the Drevliane to the west of the Dnieper, and the Severiane to the east. To their north and east were the Vyatichi, and to their south was forested land settled by Slav farmers, giving way to steppelands populated by nomadic herdsmen.

Controversy persists over whether the Rus' were Varangians or Slavs, with the current scholarly consensus holding that they were an ancestrally Norse people that quickly assimilated into Slavic culture. This uncertainty is due largely to a paucity of contemporary sources. Attempts to address this question instead rely on archaeological evidence, the accounts of foreign observers, and legends and literature from centuries later. To some extent the controversy is related to the foundation myths of modern states in the region.

While Varangians were Norse traders and Vikings, some Russian and Ukrainian nationalist historians argue that the Rus' were themselves Slavs (see The Anti-Normanist debate). Normanist theories focus on the earliest written source for the East Slavs, the Primary Chronicle, which was produced in the 12th century. Nationalist accounts on the other hand have suggested that the Rus' were present before the arrival of the Varangians, noting that only a handful of Scandinavian words can be found in Russian and that Scandinavian names in the early chronicles were soon replaced by Slavic names.

Nevertheless, the close connection between the Rus' and the Norse is confirmed both by extensive Scandinavian settlement in Belarus, Russia, and Ukraine and by Slavic influences in the Swedish language. Considering the linguistic arguments mounted by nationalist scholars, if the proto-Rus' were Norse, they must have quickly become nativized, adopting Slavic languages and other cultural practices.

Ahmad ibn Fadlan, an Arab traveler during the 10th century, provided one of the earliest written descriptions of the Rus': "They are as tall as a date palm, blond and ruddy, so that they do not need to wear a tunic nor a cloak; rather the men among them wear garments that only cover half of his body and leaves one of his hands free." Liutprand of Cremona, who was twice an envoy to the Byzantine court (949 and 968), identifies the "Russi" with the Norse ("the Russi, whom we call Norsemen by another name") but explains the name as a Greek term referring to their physical traits ("A certain people made up of a part of the Norse, whom the Greeks call [...] the Russi on account of their physical features, we designate as Norsemen because of the location of their origin."). Leo the Deacon, a 10th-century Byzantine historian and chronicler, refers to the Rus' as "Scythians" and notes that they tended to adopt Greek rituals and customs. But 'Scythians' in Greek parlance is used predominantly as a generic term for nomads.