Vukašin Mrnjavčević (Serbian Cyrillic: Вукашин Мрњавчевић, pronounced [ʋukǎʃin mr̩̂ɲaːʋtʃeʋitɕ] ⓘ; c. 1320 – 26 September 1371) was King of Serbia as the co-ruler of Stefan Uroš V from 1365 to 1371.

According to 17th-century Ragusan historian Mavro Orbini, his father was a minor noble named Mrnjava from Zachlumia, whose sons Vukašin and Uglješa Mrnjavčević were born in Livno in western Bosnia.

[3] Some of Orbin's historical accounts are based on oral traditions of his time, but a 1280 Ragusan document mentions a Mrnjan as a nobleman from Trebinje, a town in Travunia in the vicinity of Zachlumia.

Possibly the family supported Serbian Emperor Dušan's invasion of Bosnia in 1350, as did other Zachlumian nobles, and fearing punishment, emigrated to Serbia when the war was about to start.

By 1369, as Uroš was childless, Vukašin designated his eldest son Prince Marko as heir to the throne, with the title of "young king".