Stephen Tvrtko I (Serbo-Croatian: Stjepan/Stefan Tvrtko / Стјепан/Стефан Твртко; c. 1338 – 10 March 1391) was the first king of Bosnia. A member of the House of Kotromanić, he succeeded his uncle Stephen II as Ban of Bosnia in 1353. As he was a minor at the time, Tvrtko's father, Vladislav, briefly ruled as regent, followed by Tvrtko's mother, Jelena. Early in his personal rule, Tvrtko quarreled with his country's Roman Catholic clergy, but later enjoyed cordial relations with all the religious communities in his realm. After initial difficulties – the loss of large parts of Bosnia to his overlord, King Louis I of Hungary, and being briefly deposed by his magnates – Tvrtko's power grew considerably. He conquered some remnants of the neighbouring Serbian Empire in 1373, after the death of its last ruler and his distant relative, Uroš the Weak. In 1377, he had himself crowned king of Bosnia and of Serbia, claiming to be the heir of Serbia's extinct Nemanjić dynasty.

As the Kingdom of Bosnia continued to expand, Tvrtko's attention shifted to the Adriatic coast. He gained control of the entire Pomorje and the major maritime cities of the area, established new settlements and started building a navy, but never succeeded in subjugating the lords of the independent Serbian territories. The death of King Louis and the accession of Queen Mary in 1382 allowed Tvrtko to take advantage of the ensuing succession crisis in Hungary and Croatia. After bitter fighting, from 1385 to 1390, Tvrtko succeeded in conquering large parts of Slavonia, Dalmatia, and Croatia proper. Following the Battle of Kosovo in 1389, his tenuous claim to Serbia became a mere fiction, as the Serbian rulers he sought to subdue became vassals of the victorious Ottoman Empire. The Ottoman Turks also launched their first attacks on Bosnia during Tvrtko's reign, but his army was able to repel them. Tvrtko's sudden death in 1391 prevented him from solidifying the Kotromanić hold on Croatian lands.

Tvrtko is widely considered one of Bosnia's greatest medieval rulers, having enlarged the country's borders to their greatest extent, left a strong economy, and improved the living standards of his subjects. He was survived by at least one son, Tvrtko II, but was succeeded by Dabiša, under whom Tvrtko's burgeoning realm began to decay.