Peter I of Serbia (Serbian: Petar I Karađorđević; Cyrillic: Петар I Карађорђевић) reigned as the last King of Serbia (1903–1918) and as the first King of the Serbs, Croats and Slovenes (1918–1921).
Peter was Karađorđe's grandson and third son of Persida Nenadović and Prince Alexander Karađorđević, who was forced to abdicate. Peter lived with his family in exile. He fought with the French Foreign Legion in the Franco-Prussian War. He joined as volunteer under the alias Peter Mrkonjić in Herzegovina Uprising (1875–77) against Ottoman Empire.
He married Princess Zorka of Montenegro, daughter of King Nicholas I, in 1883. She gave birth to his five children, including Prince Alexander. After death of his father in 1885, Peter became head of Karađorđević dynasty. After a military coup d'état and murder of King Alexander I Obrenović in 1903, Peter became King of Serbia. As a king he advocated for constitutional setup of the country and was famous for his libertarian politics.
King Peter was a supreme commander of the Serbian army in Balkan wars. Because of his age on 24 June 1914, he proclaimed his son, Alexander, heir to the throne, as regent. In First World War he and his army retreated across Albania. Since he was a king of Serbia during a period of great Serbian military success, he was remembered by Serbian people as King Peter the Liberator (Kralj Petar oslobodilac).