Marko Mrnjavčević (Serbian Cyrillic: Марко Мрњавчевић, pronounced [mâːrko mr̩̂ɲaːʋt͡ʃeʋit͡ɕ] ^ⓘ; c. 1335 – 17 May 1395) was the de jure Serbian king from 1371 to 1395, while he was the de facto ruler of territory in western Macedonia centered on the town of Prilep. He is known as Prince Marko (Macedonian: Kрaле Марко; Serbian Cyrillic: Краљевић Марко, Kraljević Marko, IPA: [krǎːʎeʋit͡ɕ mâːrko]) and King Marko (Macedonian: Kрaл Марко; Serbian Cyrillic: Краљ Марко; Bulgarian: Крали Марко) in South Slavic oral tradition, in which he has become a major character during the period of Ottoman rule over the Balkans. Marko's father, King Vukašin, was co-ruler with Serbian Tsar Stefan Uroš V, whose reign was characterised by weakening central authority and the gradual disintegration of the Serbian Empire. Vukašin's holdings included lands in north-western Macedonia and Kosovo. In 1370 or 1371, he crowned Marko "young king"; this title included the possibility that Marko would succeed the childless Uroš on the Serbian throne.

On 26 September 1371, Vukašin was killed and his forces defeated in the Battle of Maritsa. About two months later, Tsar Uroš died. This formally made Marko the king of the Serbian land; however, Serbian noblemen, who had become effectively independent from the central authority, did not even consider to recognise him as their supreme ruler. Sometime after 1371, he became an Ottoman vassal; by 1377, significant portions of the territory he inherited from Vukašin were seized by other noblemen. King Marko, in reality, came to be a regional lord who ruled over a relatively small territory in western Macedonia. He funded the construction of the Monastery of Saint Demetrius near Skopje (better known as Marko's Monastery), which was completed in 1376. Later, Marko became an Ottoman vassal and died on 17 May 1395, fighting against the Wallachians in the Battle of Rovine.

Although a ruler of modest historical significance, Marko became a major character in South Slavic oral tradition. He is venerated as a national hero by the Serbs, Macedonians and Bulgarians, remembered in Balkan folklore as a fearless and powerful protector of the weak, who fought against injustice and confronted the Turks during the Ottoman occupation.

Marko was born about 1335 as the first son of Vukašin Mrnjavčević and his wife Alena. The patronymic "Mrnjavčević" derives from Mrnjava, described by 17th-century Ragusan historian Mavro Orbin as a minor nobleman from Zachlumia (in present-day Herzegovina and southern Dalmatia). According to Orbin, Mrnjava's sons were born in Livno in western Bosnia, where he may have moved after Zachlumia was annexed from Serbia by Bosnia in 1326. The Mrnjavčević family^n.b.1 may have later supported Serbian Emperor (tsar) Stefan Dušan in his preparations to invade Bosnia as did other Zachlumian nobles, and, fearing punishment, emigrated to the Serbian Empire before the war started. These preparations possibly began two years ahead of the invasion, which took place in 1350. From that year comes the earliest written reference to Marko's father Vukašin, describing him as Dušan's appointed župan (district governor) of Prilep, which was acquired by Serbia from Byzantium in 1334 with other parts of Macedonia. In 1355, at about age 47, Stefan Dušan died suddenly of a stroke.

Dušan was succeeded by his 19-year-old son Uroš, who apparently regarded Marko Mrnjavčević as a man of trust. The new Emperor appointed him the head of the embassy he sent to Ragusa (now Dubrovnik, Croatia) at the end of July 1361 to negotiate peace between the empire and the Ragusan Republic after hostilities earlier that year. Although peace was not reached, Marko successfully negotiated the release of Serbian merchants from Prizren who were detained by the Ragusans and was permitted to withdraw silver deposited in the city by his family. The account of that embassy in a Ragusan document contains the earliest-known, undisputed reference to Marko Mrnjavčević. An inscription written in 1356 on a wall of a church in the Macedonian region of Tikveš, mentions a Nikola and a Marko as governors in that region, but the identity of this Marko is disputed.

Dušan's death was followed by the stirring of separatist activity in the Serbian Empire. The south-western territories, including Epirus, Thessaly, and lands in southern Albania, seceded by 1357. However, the core of the state (the western lands, including Zeta and Travunia with the upper Drina Valley; the central Serbian lands; and Macedonia), remained loyal to Emperor Uroš. Nevertheless, local noblemen asserted more and more independence from Uroš' authority even in the part of the state that remained Serbian. Uroš was weak and unable to counteract these separatist tendencies, becoming an inferior power in his own domain. Serbian lords also fought each other for territory and influence.

Vukašin Mrnjavčević was a skilful politician, and gradually assumed the main role in the empire. In August or September 1365 Uroš crowned him king, making him his co-ruler. By 1370, Marko's potential patrimony increased as Vukašin expanded his personal holdings from Prilep further into Macedonia, Kosovo and Metohija, acquiring Prizren, Pristina, Novo Brdo, Skopje and Ohrid. In a charter he issued on 5 April 1370, Vukašin mentioned his wife (Queen Alena) and sons (Marko and Andrijaš), signing himself as "Lord of the Serb and Greek Lands, and of the Western Provinces" (господинь зємли срьбьскои и грькѡмь и западнимь странамь). In late 1370 or early 1371, Vukašin crowned Marko "Young King", a title given to heirs presumptive of Serbian kings to secure their position as successors to the throne. Since Uroš was childless, Marko could thus become his successor, beginning a new—Vukašin—dynasty of Serbian sovereigns, and ending the two-century Nemanjić dynasty. Most Serbian lords were unhappy with the situation, which strengthened their desire for independence from the central authority.

Vukašin sought a well-connected spouse for Marko. A princess from the Croatian House of Šubić of Dalmatia was sent by her father, Grgur, to the court of their relative Tvrtko I, the ban of Bosnia. She was supposed to be raised and married by Tvrtko's mother Jelena. Jelena was the daughter of George II Šubić, whose maternal grandfather was Serbian King Dragutin Nemanjić. The ban and his mother approved of Vukašin's idea to join the Šubić princess and Marko, and the wedding was imminent. However, in April 1370 Pope Urban V sent Tvrtko a letter forbidding him to give the Catholic lady in marriage to the "son of His Magnificence, the King of Serbia, a schismatic" (filio magnifici viri Regis Rascie scismatico). The pope also notified King Louis I of Hungary, nominal overlord of the ban, of the impending "offence to the Christian faith", and the marriage did not occur. Marko subsequently married Jelena (daughter of Radoslav Hlapen, the lord of Veria and Edessa and the major Serbian nobleman in southern Macedonia).