Tvrtko Kotromanic (1338?-1391) was a ruler of Bosnia. He was the son of Vladislav Kotromanic and Jelena Šubic. Tvrtko became Ban of Bosnia in 1353 upon the death of his uncle, Ban Stephen Kotromanic, when Tvrtko was 15 years old. During the first part of his reign as Ban he had to contend with intrigues, revolts, and seizures of Bosnian territory by Hungary. In 1366 he was forced to seek refuge in the Hungarian court when a group of Bosnian nobles placed his brother Vuk on the throne. He was restored as Ban the following year with the assistance of the King Louis I of Hungary.
Tvrtko assisted Prince (Knez in Serbian) Lazar Hrebeljanovic, in neighboring Serbia, in consolidating his control of the Serb territories to the east; in return, Tvrtko was able to expand his own territory to include parts of Hum (Herzegovina), Zeta (Montenegro), and Serb territories to the east of the present Bosnian border, including the monastery of Milesevo, an important site to Serbs because it contains the relics of Saint Sava, an central figure in the history of the Serbian Orthodox Church. Tvrtko had himself crowned King of Bosnia and Serbia at Milesevo in 1377. Although he had declared himself King of Serbia, was descended from the founder of Serbia's Nemanjid dynasty, and had created a Byzantine-style court at his capital at Bobovac, he only controlled a small part of Serbia, and made no serious attempts to extend his kingdom further east into Serb lands.
Tvrtko did continue to expand his kingdom to the south and west. His predecessor, Ban Stephen Kotromanic, had added part of the Dalmatian coastline between Ragusa (Dubrovnik) and Split to the Bosnian Kingdom, and Tvrtko expanded northwards and southwards along the coast, from south of Zara (modern Zadar to the Bay of Kotor, with the exception of Ragusa, which remained independent. He established the port of Novi (modern Herceg Novi on the Bay of Kotor, and in the last few years of his reign called himself King of Croatia and Dalmatia too.
Tvrtko died in 1391, and by the end of his reign the Medieval Bosnian state reached its greatest power and territorial extent. Upon his death he was succeeded by his son, Ostoja Kotromanic.