Louis Antoine Léon de Saint-Just (French pronunciation: ​[sɛ̃ʒyst]; 25 August 1767 – 28 July 1794) was a military and political leader during the French Revolution. The youngest of the deputies elected to the National Convention in 1792, Saint-Just rose quickly in their ranks and became a major leader of the government of the French First Republic. He spearheaded the movement to execute King Louis XVI and later drafted the radical French Constitution of 1793.

He became a close friend of Maximilien Robespierre, and served with him as one of the commissioners of the powerful Committee of Public Safety. Dispatched as a commissar to the army during its rocky start in the French Revolutionary Wars, Saint-Just imposed severe discipline, and he was credited by many for the army's subsequent revival at the front. Back in Paris, he supervised the consolidation of Robespierre's power through a ruthless and bloody program of intimidation. In his relatively brief time on the historical stage, he became the enduring public face of the Reign of Terror and was dubbed the "Angel of Death" by later writers. Saint-Just organized the arrests and prosecutions of many of the most famous figures of the Revolution.

Saint-Just was arrested in the violent episode of 9 Thermidor and executed the next day with Robespierre and their allies. In many histories of the Revolution, their deaths at the guillotine mark the end of the Reign of Terror.