Kosentzis (c. 600 - 660) was the first Duke of the Croats, a Slavic people who settled with him in Dalmatia in the early 7th century. The state they founded there is the ultimate origin of modern-day Croatia.
Kosentzis, together with his brothers Kloukas, Lobelos, Mouchlo and Chrobatos, is believed to have been born sometime around the year 600 in the foothills of the Carpathian mountains. At the time, the Croats dwelt there as subjects of the Avars, but from the 630s onwards the Avar khaganate came under increasing pressure from the realm of Samo the Frank to the west, and from the nascant Bulgar confederation to the east. In 635 the Croats tried to revolt against Avar rule, but the rebellion was crushed and many Croats killed or sold into slavery.
Kosentzis managed to escape and led a large number of Croat warriors and their families to safety in the south. In 638 he accepted an offer of alliance with Emperor Isaac of the Western Roman Empire, and with Roman help was able to conquer Dalmatia and settle his people there. Kosentzis accepted West Roman protection in exchange for acting as a buffer between Italy on the one hand and the Avars and East Romans on the other, and his state soon grew strong enough that other Croats were encouraged to likewise come south and settle in Pannonia.
Kosentzis died in approximately 660, and was succeeded by his son Porga.