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Bosnia reached its peak under Ban Tvrtko I, a member of the Kotromanić dynasty, who came to power in 1353. In 1372, Tvrtko formed an alliance with Prince Lazar Hrebeljanović, one of the regional lords in the territory of the disintegrated Serbian Empire. The next year, Tvrtko and Lazar attacked the domain of Nikola Altomanović, who was the most powerful Serbian noble at the time. After defeating Altomanović, they divided his lands, except for his littoral districts of Dračevica, Konavle, and Trebinje, which were seized by Đurađ I Balšić, the Lord of Zeta. Tvrtko received parts of Zahumlje, the upper reaches of the Drina and Lim rivers, as well as the districts of Onogošt and Gacko. This acquisition included the important Serbian Orthodox monastery of Mileševa (which held the relics of Saint Sava, the first Serbian Archbishop).

In 1377, Tvrtko took the littoral districts from Balšić. That year, on 26 October, he was crowned King of 'Serbia, Bosnia, the Primorje (Seaside), and the western lands'. The acquisition of Serbian territory, including the important Monastery of Mileševa, combined with the fact that Tvrtko's grandmother had been a member of the Nemanjić dynasty prompted Tvrtko into also having himself crowned 'King of Serbia' thus asserting his pretensions to the Serbian throne. This was made possible by the royal Nemanjić line having died out with Uroš in 1371. The crown was sent to him by Hungarian king Louis of Anjou. According to plurality of recent works, in historiography represented by scholars like Čošković, Anđelić, Lovrenović, Filipović, ceremony itself was conducted in Mile near Visoko in the church which was built in time of Stephen II Kotromanić's reign, where he was also buried alongside his uncle Stjepan II. In contrast, some earlier historiographers, mostly represented by scholars from Serbia, consider that he was crowned in the Orthodox Monastery of Mileševa, by the Metropolitan of Mileševa.

However, after the defeat of Nikola Altomanović, Prince Lazar had emerged as the most powerful lord on the territory of the former Serbian Empire. He wanted to reunite the Serbian state, and the Serbian Orthodox Church saw him as most fitted to succeed the Nemanjić dynasty. The Church, which was the strongest cohesive force among the Serbs at the time, did not support Tvrtko's aspirations in this regard.

By 1390, Tvrtko I expanded his realm to include a part of Croatia and Dalmatia, and expanded his title to King of Rascia, Bosnia, Dalmatia, Croatia and the Littoral. Tvrtko's full title listed subject peoples and geographical dependencies, following the Byzantine norm. At the peak of his power, he was "King of Bosnia, Serbia, Croatia, Hum, Usora, Soli, Dalmatia, Donji Kraji".