Alexander I Karađorđević (Serbo-Croatian: Александар I Карађорђевић, romanized: Aleksandar I Karađorđević, pronounced [aleksǎːndar př̩viː karadʑǒːrdʑevitɕ]; 16 December 1888 [O.S. 4 December] – 9 October 1934), also known as Alexander the Unifier (Aleksandar Ujedinitelj / Александар Ујединитељ [aleksǎːndar ujedǐniteʎ]), was King of the Serbs, Croats and Slovenes from 16 August 1921 to 3 October 1929 and King of Yugoslavia from 3 October 1929 until his assassination in 1934. His reign of 13 years is the longest of the three monarchs of the Kingdom of Yugoslavia.

Born in Cetinje, Montenegro, Alexander was the second son of Peter and Zorka Karađorđević. The House of Karađorđević had been removed from power in Serbia 30 years prior, and Alexander spent his early life in exile with his father in Montenegro and then Switzerland. Afterwards he moved to Russia and enrolled in the imperial Page Corps. Following a coup d'état and the murder of King Alexander I Obrenović in 1903, his father became King of Serbia. In 1909, Alexander's elder brother, George, renounced his claim to the throne, making Alexander heir apparent. Alexander distinguished himself as a commander during the Balkan Wars, leading the Serbian army to victory over the Ottoman Turks and the Bulgarians. In 1914, he became prince regent of Serbia. During the First World War, he held nominal command of the Royal Serbian Army.

In 1918, Alexander oversaw the unification of Serbia and the former Austrian provinces of Croatia-Slavonia, Slovenia, Vojvodina, Bosnia and Herzegovina, and Dalmatia into the Kingdom of Serbs, Croats and Slovenes on the basis of the Corfu Declaration. He ascended to the throne upon his father's death in 1921. An extended period of political crisis followed, culminating in the assassination of Croat leader Stjepan Radić. In response, Alexander abrogated the Vidovdan Constitution in 1929, prorogued the parliament, changed the name of the country to Kingdom of Yugoslavia and established a royal dictatorship. The 1931 Constitution formalised Alexander's personal rule and confirmed Yugoslavia's status as a unitary state, further aggravating the non-Serb population. Political and economic tensions escalated on the outbreak of the Great Depression, which devastated the predominantly rural country. In foreign affairs, Alexander supported the Balkan Pact with Greece, Romania, and Turkey, and sought to improve relations with Bulgaria.

In 1934, Alexander embarked on a state visit to France in order to secure support for the Little Entente against Hungarian revanchism and Fascist Italy's imperialist designs. During a stop in Marseille, he was assassinated by Vlado Chernozemski, a member of the pro-Bulgarian Internal Macedonian Revolutionary Organization, which received assistance from the Croat Ustaše led by Ante Pavelić. French Foreign Minister Louis Barthou also died in the attack. Alexander was succeeded by his eleven-year-old son, Peter II, under the regency of his first cousin Prince Paul.

Alexander Karađorđević was born on 16 December 1888 [O.S. 4 December] in the Principality of Montenegro as the fourth child and second son of Peter Karađorđević and his wife, Princess Zorka of Montenegro. His paternal grandfather, also named Alexander, had been forced to abdicate as prince of Serbia and surrender power to the rival House of Obrenović. Alexander's maternal grandfather was Nicholas I, Prince of Montenegro. Despite enjoying support from the Russian Empire, at the time of Alexander's birth and early childhood, the House of Karađorđević was in political exile, with family members scattered all over Europe, unable to return to Serbia.

Serbia had recently been transformed from a principality into a kingdom under the Obrenovićs, who ruled with strong support from the Austro-Hungarian Empire. The antagonism between the two rival royal houses was such, that, after the assassination of Prince Mihailo Obrenović in 1868 (an event Karađorđevićs were suspected of taking part in), the Obrenovićs resorted to making constitutional changes, specifically proclaiming the Karađorđevićs banned from entering Serbia and stripping them of their civic rights.

Alexander was two when his mother, Princess Zorka, died in 1890 from complications while giving birth to his younger brother, Andrej, who died 23 days later. Alexander spent his childhood in Montenegro. In 1894, his widower father took the four children, including Alexander, to Geneva, Switzerland where the young man completed his elementary education.^{[citation needed]} Alongside his older brother George, he continued his schooling at the imperial Page Corps in Petrograd, Russian Empire. The British historian Robert Seton-Watson described Alexander as becoming a Russophile during his time in Petrograd, feeling much gratitude for the willingness of the Russian Emperor Nicholas II to give him a refuge, where he was treated with much honor and respect.

As a page, Alexander was described as hard-working and determined while also being a "loner" who kept to himself and rarely showed his feelings. Being a Karađorđević led to Alexander being invited by the Russian Emperor Nicholas II to dinner at the Winter Palace, where he was the guest of honor at meals hosted by the Russian imperial family, which was a great honor for a prince from Serbia's deposed princely family. During his time in Petrograd, Alexander visited the Alexander Nevsky Monastery, where the abbot gave Alexander an icon of Prince Alexander Nevsky and guided him to the grave of Marshal Alexander Suvorov. After his visit to the monastery, Alexander expressed the wish to be a great general like Marshal Suvorov or Prince Alexander Nevsky, saying he wanted to be commanding either a great army or a great armada when he was a man.