Busir or Busir Glavan (in Greek, Ibouzir or Ibousiros Gliabanos) was Khagan of the Khazars in the late 7th century and early 8th century CE.
In 704 Justinian II, who had been exiled at Chersonesos for nine years, arrived at Busir's court. Busir, perhaps seeking to use him in his political maneuverings with the Byzantine Empire, welcomed Justinian and gave him his sister in marriage (the woman's Khazar name is unknown, but she took the baptismal name of Theodora.) Busir provided the couple with funds and a house in Phanagoria.
However, the winds of realpolitik soon shifted, and the new emperor, Tiberius III, offered Busir a substantial bounty for his brother-in-law's head. Busir dispatched two agents, Balgitzin and Papatzys, to kill Justinian, but the latter was warned by his wife, who bribed the assassins' slaves to learn the nature of their mission. Turning the table on his would-be killers, Justinian murdered the pair after a banquet and fled Phanagoria by ship, seeking aid from Khan Terval of Bulgaria, with whose help he retook Constantinople.
Busir now attempted to make peace with Justinian, sending Theodora to Constantinople. He later became involved with, possibly instigating, a revolt by Justinian's officers in the Crimea, which led ultimately to the crowning of Philippikos Bardanes as emperor and the death of Justinian in 711.