John I Tzimiskes (Greek: Ἰωάννης Α΄ Τζιμισκής) was the senior Byzantine Emperor from 11 December 969 to 10 January 976. An intuitive and successful general, he strengthened the Empire and expanded its borders during his short reign.
John I Tzimiskes was born into the Kourkouas clan, a family of Armenian origin. On the death of Emperor Romanos II in 963, Tzimiskes urged his uncle to seize the throne. After helping Nikephoros to the throne and continuing to defend the Empire's eastern provinces, Tzimiskes was deprived of his command by an intrigue, for which he retaliated by conspiring with Nikephoros' wife Theophano and a number of disgruntled leading generals (Michael Bourtzes and Leo Balantes) to assassinate Nikephoros.
After his coronation in December 969, Tzimiskes dispatched his brother-in-law Bardas Skleros to subdue a rebellion by Bardas Phokas, a cousin of Tzimiskes who aspired to succeed their uncle as emperor. To solidify his position, Tzimiskes married Theodora, a daughter of Emperor Constantine VII.
He died suddenly in 976 returning from his second campaign against the Abbasids and was buried in the Church of Christ Chalkites, which he had rebuilt.