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 the 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 quarrelled 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 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 Primorje region 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 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.

Tvrtko was the elder son of Vladislav Kotromanić and Jelena Šubić and was likely born within a year of their marriage, which was celebrated in 1337. His father was the brother of the Bosnian ban, Stephen II, and his mother the daughter of the Croatian lord George II Šubić of Bribir. Although Vladislav was still alive, Stephen's title passed directly to Tvrtko; the reason for Vladislav's exclusion from the Kotromanić succession is unclear. Tvrtko, however, was only about fifteen years old at the time, so his father governed as regent. Soon after his accession, Tvrtko travelled with his father throughout the realm to settle relations with his vassals. Jelena replaced Vladislav as regent upon his death in 1354. She immediately travelled to Hungary to obtain consent to Tvrtko's accession from King Louis I, his overlord. Following her return, Jelena held an assembly (stanak) in Mile, with mother and son confirming the possessions and privileges of the noblemen of "all of Bosnia, Donji Kraji, Zagorje, and the Hum land".

The death of Tvrtko's maternal uncle Mladen III Šubić in 1348 led to a decline of the Šubić noble family and a long conflict over their lands. In May 1355, Jelena and Tvrtko marched with an army to Duvno in order to claim Tvrtko's share of her brother's patrimony. An agreement was reached with the vice-ban of Dalmatia by which Tvrtko was to inherit all the cities held by his maternal grandfather and a city which belonged to his aunt Katarina. Still, it is unknown whether he actually took possession of them.

The state assembled by Tvrtko's uncle Stephen broke apart on Tvrtko's accession, much to the satisfaction of his overlord King Louis. The Hungarians were keen to encourage Stephen's vassals to act independently from Tvrtko, forcing Tvrtko to compete with Louis for their loyalty in order to rebuild the Bosnian state. Louis posed a more direct threat as well; he was determined to enlarge his royal domain, and throughout his realm he ardently reclaimed all lands that once belonged to the monarch. Taking advantage of the precarious situation early in Tvrtko's reign, Louis moved to claim most of Donji Kraji and western Hum up to the river Neretva, including the prosperous customs town of Drijeva. In 1357, he succeeded in compelling Tvrtko to come to Hungary and surrender these territories as the dowry of Stephen's daughter Elizabeth, who had been married to Louis since 1353. In July, King Louis confirmed Tvrtko and his younger brother Vuk as rulers of Bosnia and Usora. Donji Kraji and Hum were purposely omitted from their title, with Usora likely having been granted as compensation. Two conditions were forced upon the Bosnians: one of the two Kotromanić brothers would be at Louis's court whenever the other was in Bosnia, and they would make an effort to suppress the "heretical" Bosnian Church.

Little is known about internal affairs in Bosnia between 1357 when Tvrtko started ruling on his own and 1363. His religious policy came into focus in this period, as the Avignon papacy became more insistent on curbing the Bosnian Church. This endangered Tvrtko, for although he was a Roman Catholic throughout his life, Louis now had a religious pretext for invading Bosnia. The death of the bishop of Bosnia—Peregrin Saxon, a supporter of both Stephen II and Tvrtko I and acknowledged by the latter as his "spiritual father"—led to the appointment of Peter Siklósi to the episcopal throne. Peter actively promoted the idea of launching a new crusade against Bosnia, earning him Tvrtko's hostility. Tvrtko even attempted to plot against Peter but failed when his letters to a lector in Peter's Đakovo residence were discovered. The Bosnian Church, meanwhile, survived throughout Tvrtko's reign but only became prominent in state affairs after his death. One hostile source even tried to link Tvrtko himself to the Church due to his tolerance of all local faiths, including Hum's Eastern Orthodoxy.

At the start of his personal rule, the young Ban somehow considerably increased his power. Although he constantly emphasized his subservience and loyalty to the King, Tvrtko started regarding the loyalty of the Donji Kraji noblemen to Louis as treachery against himself. In 1363, a conflict broke out between the two men. The cause is not clear, although Louis stated that his intention was to eradicate the Bosnian heretics. By April, the King had begun amassing an army; and in May, officials of the Republic of Ragusa ordered their merchants to leave Bosnia due to an imminent clash. An army led by Louis himself attacked Donji Kraji, where the nobility was divided in its loyalties between Tvrtko and Louis. A month later an army led by the palatine of Hungary, Nicholas Kont, and the archbishop of Esztergom, Nicholas Apáti, struck Usora. Vlatko Vukoslavić deserted to Louis and surrendered to him the important fortress of Ključ, but Vukac Hrvatinić succeeded in defending the Soko Grad fortress in the župa of Pliva, forcing the Hungarians to retreat. In Usora, the Srebrenik Fortress held out against a "massive attack" by the royal army, which suffered the embarrassment of losing the King's seal. The successful defense of Srebrenik marked Tvrtko's first victory against Hungary.