The Varangian Guard was an elite unit of the Byzantine Army from the ninth to the fourteenth century who served as personal bodyguards to the Byzantine emperors. The Varangian Guard was known for being primarily composed of recruits from northern Europe, including mainly Norsemen from Scandinavia but also Anglo-Saxons from England. The recruitment of distant foreigners from outside Byzantium to serve as the emperor's personal guard was pursued as a deliberate policy, as they lacked local political loyalties and could be counted upon to suppress revolts by disloyal Byzantine factions.

The earliest members of the Varangian guard came from Kievan Rus'. A treaty between Rus' and the Byzantine empire under Basil I was agreed in 874 after a period of hostilities. A clause in the treaty obliged Rus' to provide men for Byzantine service. Renewed hostilities between 907 and 911 ended with a new treaty under which any Rus' who chose could serve Byzantium as a right.

The Rus' provided the earliest members of the Varangian Guard. They were in Byzantine service from as early as 874. The Guard was first formally constituted under Emperor Basil II in 988, following the Christianization of Kievan Rus' by Vladimir I of Kiev. Vladimir, who had recently usurped power in Kiev with an army of Varangian warriors, sent 6,000 men to Basil as part of a military assistance agreement. Basil's distrust of the native Byzantine guardsmen, whose loyalties often shifted with fatal consequences, as well as the proven loyalty of the Varangians, many of whom had previously served in Byzantium, led the Emperor to employ them as his personal guardsmen.

Immigrants from Scandinavia (predominantly immigrants from Sweden, but also elements from Denmark and Norway) kept an almost entirely Norse cast to the organization until the late 11th century. According to the late Swedish historian Alf Henrikson in his book Svensk Historia (History of Sweden), the Norse Varangian guardsmen were recognized by long hair, a red ruby set in the left ear and ornamented dragons sewn on their chainmail shirts. During these years, Swedish men left to enlist in the Byzantine Varangian Guard in such numbers that a medieval Swedish law, Västgötalagen, from Västergötland declared no one could inherit while staying in "Greece"

Composed primarily of Norsemen and Rus for the first 100 years, the Guard began to see increased numbers of Anglo-Saxons after the Norman conquest of England. By the time of the Emperor Alexios Komnenos in the late 11th century, the Varangian Guard was largely recruited from Anglo-Saxons and others who had suffered at the hands of the Vikings and their cousins the Normans.