John I Tzimiskes (Greek: Ἰωάννης ὁ Τζιμισκής, romanized: Iōánnēs ho Tzimiskēs; c. 925 – 10 January 976) was the senior Byzantine emperor from 969 to 976.

An intuitive and successful general who married into the influential Skleros family, he strengthened and expanded the Byzantine Empire to include Thrace and Syria by warring with the Rus' under Sviatoslav I and the Fatimids respectively.

[10] A more favorable explanation is offered by the medieval Armenian historian Matthew of Edessa, who states that Tzimiskes was from the region of Khozan, from the area called Chmushkatzag.

[13] Tzimiskes was born in 924 or 925, as Leo the Deacon states that he died aged 51,[14] to an unnamed member of the Kourkouas family and the sister of the future Emperor Nikephoros II Phokas.

Contemporary sources describe Tzimiskes as a rather short but well-built man, with reddish blonde hair and beard and blue eyes who was attractive to women.

Haemus, and besieged the fortress of Dorostolon (Silistra) on the Danube for 65 days, where after several hard-fought battles he defeated Grand Prince Sviatoslav I of Kiev.

On his return to Constantinople, Tzimiskes celebrated a triumph, expanded the Church of Christ of the Chalke as thanksgiving, divested the captive Bulgarian Emperor Boris II of the Imperial symbols, and proclaimed Bulgaria annexed.

A second campaign, in 975, was aimed at Syria, where his forces took Emesa, Heliopolis, Damascus, Tiberias, Nazareth, Caesarea, Sidon, Beirut, Byblos, and Tripoli, but failed to take Jerusalem.

[20] Finnish philologist and researcher Paavo Hohti asserts that Tzimiskes was one of "Byzantine's most capable military generals", noting his talents as a mediator and a reformer of religious institutions.

According to Hohti, Tzimiskes' successful campaigns against the Rus and the Arabs allowed him to restore the ascendancy of the Eastern Roman Empire in the Balkans and Mesopotamia.