The St. Bartholomew's Day massacre (Massacre de la Saint-Barthélemy in French) is a wave of Catholic mob violence against the Huguenots (French Calvinist Protestants), during the French Wars of Religion.

Traditionally believed to have been instigated by Catherine de' Medici, the mother of King Charles IX, the massacre takes place six days after the wedding of the king's sister to the Protestant Henry of Navarre, an occasion for which many of the most wealthy and prominent Huguenots have gathered in largely Catholic Paris.

Events begin two days after the attempted assassination of Admiral Gaspard de Coligny, a Huguenot military leader.

Starting on 24 August 1572 (the feast of Bartholomew the Apostle) with the murder of Coligny, the massacres spread throughout Paris, and later to other cities and the countryside, lasting for several months.

The exact number of fatalities is not known, but it is estimated that anywhere from ten thousand to possibly one-hundred thousand Huguenots die in the violence throughout France.

Though by no means unique, "it was the worst of the century's religious massacres."

The massacres mark a turning-point in the French Wars of Religion.

The Huguenot political movement is crippled by the loss of many of its prominent aristocratic leaders, and those who remain are increasingly radicalized.