Vladimir I Sviatoslavich or Volodymyr I Sviatoslavych (Old East Slavic: Володимѣръ Свѧтославичь, romanized: Volodiměr Svętoslavič; Christian name: Basil; c. 958 – 15 July 1015), given the epithet "the Great", was Prince of Novgorod from 970 and Grand Prince of Kiev from 978 until his death in 1015. The Catholic Church and the Eastern Orthodox Church both canonised him as Saint Vladimir.

Vladimir's father was Sviatoslav I of the Rurik dynasty. After the death of his father in 972, Vladimir, who was then the prince of Novgorod, was forced to flee abroad after his brother Yaropolk murdered his other brother Oleg in 977 to become the sole ruler of Rus'. Vladimir assembled a Varangian army and returned to depose Yaropolk in 978. By 980, Vladimir had consolidated his realm to the Baltic Sea and solidified the frontiers against incursions of Bulgarians, Baltic tribes and Eastern nomads. Originally a follower of Slavic paganism, Vladimir converted to Christianity in 988, and Christianized the Kievan Rus.

Several scholars refer to Vladimir as Volodimer, also spelled Volodimir, and his descendants as Volodimerovichi (sometimes in lieu of "Rurikids"). In the history of Scandinavia, Vladimir is also known as Valdemar or the Old Norse form Valdamarr (see Waldemar).

Born in 958, Vladimir was the illegitimate and youngest son of Sviatoslav I of Kiev by his housekeeper Malusha. Malusha is described in the Norse sagas as a prophetess who lived to the age of 100 and was brought from her cave to the palace to predict the future. Malusha's brother Dobrynya was Vladimir's tutor and most trusted advisor. Hagiographic tradition of dubious authenticity also connects his childhood with the name of his grandmother, Olga of Kiev, who was Christian and governed the capital during Sviatoslav's frequent military campaigns.

Transferring his capital to Pereyaslavets, Sviatoslav designated Vladimir ruler of Novgorod the Great in 970, but gave Kiev to his legitimate son Yaropolk. After Sviatoslav's death at the hands of the Pechenegs in 972, a fratricidal war erupted in 977 between Yaropolk and his younger brother Oleg, ruler of the Drevlians; Vladimir fled abroad and assembled a Varangian army to assist him in deposing Yaropolk, against whom he marched on his return the next year. On his way to Kiev he sent ambassadors to Rogvolod (Norse: Ragnvald), prince of Polotsk, to sue for the hand of his daughter Rogneda (Norse: Ragnhild). The high-born princess refused to affiance herself to the son of a bondswoman (and was betrothed to Yaropolk), so Vladimir attacked Polotsk, took Ragnhild by force, and put her parents to the sword. Polotsk was a key fortress on the way to Kiev, and capturing it along with Smolensk facilitated the taking of Kiev in 978, where he slew Yaropolk by treachery and was proclaimed knyaz of all Kievan Rus'.

Vladimir continued to expand his territories beyond his father's extensive domain. In 981, he seized the Cherven towns from the Duchy of Poland; in 981–982, he suppressed a Vyatichi rebellion; in 983, he subdued the Yatvingians; in 984, he conquered the Radimichs; and in 985, he conducted a military campaign against the Volga Bulgars, planting numerous fortresses and colonies on his way.

Although Christianity had spread in the region under Oleg's rule, Vladimir had remained a thoroughgoing pagan, taking eight hundred concubines (along with numerous wives) and erecting pagan statues and shrines to gods.

He may have attempted to reform Slavic paganism in an attempt to identify himself with the various gods worshipped by his subjects. He built a pagan temple on a hill in Kiev dedicated to six gods: Perun—the god of thunder and war, a god favored by members of the prince's druzhina (military retinue); Slavic gods Stribog and Dazhd'bog; Mokosh—a goddess representing Mother Nature "worshipped by Finnish tribes"; Khors and Simargl, "both of which had Iranian origins, were included, probably to appeal to the Poliane".