Alexander I (Serbian Cyrillic: I , romanized: Aleksandar I Karaorevi, pronounced [aleksndar pi karadrdeit]) (16 December 1888 [O.S. 4 December] 9 October 1934), also known as Alexander the Unifier, was the prince regent of the Kingdom of Serbia from 1914 and later the King of Yugoslavia from 1921 to 1934 (prior to 1929 the state was known as the Kingdom of Serbs, Croats and Slovenes). He was assassinated by the Bulgarian Vlado Chernozemski of the Internal Macedonian Revolutionary Organization, during a 1934 state visit to France. Having sat on the throne for 13 years, he is the longest-reigning monarch of the Kingdom of Yugoslavia.

Regicide is the purposeful killing of a monarch or sovereign of a polity and is often associated with the usurpation of power. A regicide can also be the person responsible for the killing. The word comes from the Latin roots of regis and cida (cidium), meaning "of monarch" and "killer" respectively.

In the British tradition, it refers to the judicial execution of a king after a trial, reflecting the historical precedent of the trial and execution of Charles I of England. The concept of regicide has also been explored in media and the arts through pieces like Macbeth (Macbeth's killing of King Duncan) and The Lion King.