Prior to the emergence of Kievan Rus' in the 9th century, most of the area north of the Black Sea, which roughly overlaps with modern-day Ukraine and Belarus, was 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 steppe lands populated by nomadic herdsmen.

There was once controversy over whether the Rus' were Varangians or Slavs, however, more recently scholarly attention has focused more on debating how quickly an ancestrally Norse people 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. This often unfruitful debate over origins has periodically devolved into competing nationalist narratives of dubious scholarly value being promoted directly by various government bodies, in a number of states. This was seen in the Stalinist period, when Soviet historiography sought to distance the Rus' from any connection to Germanic tribes, in an effort to dispel Nazi propaganda claiming the Russian state owed its existence and origins to the supposedly racially superior Norse tribes. More recently, in the context of resurgent nationalism in post-Soviet states, Anglophone scholarship has analyzed renewed efforts to use this debate to create ethno-nationalist foundation stories, with governments sometimes directly involved in the project. Conferences and publications questioning the Norse origins of the Rus' have been supported directly by state policy in some cases, and the resultant foundation myths have been included in some school textbooks in Russia.

While Varangians were Norse traders and Vikings, some Russian and Ukrainian nationalist historians argue that the Rus' were themselves Slavs (see Anti-Normanism). 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 settlements in Belarus, Russia, and Ukraine and by Slavic influences in the Swedish language.Though the debate over the origin of the Rus' remains politically charged, there is broad agreement that if the proto-Rus' were indeed originally Norse, they were quickly nativized, adopting Slavic languages and other cultural practices. This position, roughly representing a scholarly consensus (at least outside of nationalist historiography), was summarized by the historian, F. Donald Logan, "in 839, the Rus were Swedes; in 1043 the Rus were Slavs". Recent scholarship has attempted to move past the narrow and politicized debate on origins, to focus on how and why assimilation took place so quickly. Some modern DNA testing also points to Viking origins, not only of some of the early Rus' princely family and/or their retinues but also links to possible brethren from neighboring countries like Sviatopolk I of Kiev.

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 callthe 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.
Summarise the history of the origin of the Kievan Rus' name
The origin of the name the Kievan Rus' has a lot of debates, especially in some countries with strong influence on nationalist ideas(for more information check ethno-nationalist foundation stories). There was once controversy over whether the Rus' were Varangians or Slavs.  Some historians argue that the Rus' were themselves Slavs, while Varangians were Norse traders and Vikings. The close connection between the Rus' and the Norse is confirmed both by extensive Scandinavian settlements that were found on nowadays territories of  Belarus, Russia, and Ukraine and by Slavic influences in the Swedish language.

Putting aside the pro-nationalist politico-historical view, we could follow historical and archeological findings that explain tribes' formations on the territory that was later names, Kievan Rus'.  

Liutprand of Cremona, identifies the "Russi" with the Norse but explains the name as a Greek term referring to their physical traits. 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.

Before the Kievan Rus' creation or appearance, most of the area north of the Black Sea was primarily populated by eastern Slavic tribes. But it also was surrounded and populated by many more as Ilmen Slavs and neighboring Krivich, Finnic Chud tribe,  Poliane, a group of Slavicized tribes with Iranian origins, the Drevliane and the Severiane together with the Vyatichi, and to their south was forested land settled by Slav farmers, giving way to steppe lands populated by nomadic herdsmen.