The St. Bartholomew's Day massacre (French: Massacre de la Saint-Barthélemy) in 1572 was a targeted group of assassinations and a wave of Catholic mob violence directed against the Huguenots (French Calvinist Protestants) during the French Wars of Religion. Traditionally believed to have been instigated by Queen Catherine de' Medici, the mother of King Charles IX, the massacre started a few days after the marriage on 18 August of the king's sister Margaret to the Protestant King Henry III of Navarre. Many of the wealthiest and most prominent Huguenots had gathered in largely Catholic Paris to attend the wedding.

The massacre began in the night of 23–24 August 1572, the eve of the feast of Bartholomew the Apostle, two days after the attempted assassination of Admiral Gaspard de Coligny, the military and political leader of the Huguenots. King Charles IX ordered the killing of a group of Huguenot leaders, including Coligny, and the slaughter spread throughout Paris. Lasting several weeks in all, the massacre expanded outward to the countryside and other urban centres. Modern estimates for the number of dead across France vary widely, from 5,000 to 30,000.

The massacre marked a turning point in the French Wars of Religion. The Huguenot political movement was crippled by the loss of many of its prominent aristocratic leaders, and many rank-and-file members subsequently converted. Those who remained became increasingly radicalized. Though by no means unique, the bloodletting "was the worst of the century's religious massacres". Throughout Europe, it "printed on Protestant minds the indelible conviction that Catholicism was a bloody and treacherous religion".