St. Bartholomew's Day Massacre, (August 24, 1572), the slaying in France of thousands of Huguenots (French Protestants) during the religious conflict of the 16th century. The massacre, instigated by the queen mother Catherine de' Medici and sanctioned by the young king, Charles IX, was a severe blow to French Protestantism. However, its ultimate result was to strengthen the resistance of surviving Huguenots to Catholic pressures, and open religious warfare was soon renewed.

During the 16th century, Protestantism spread rapidly in France. Hostility between Catholics and Huguenots developed into civil war in 1562. The extreme Catholic party was led by members of the powerful ducal family of Guise. Prominent among Huguenot leaders was Gaspard de Coligny, Admiral of France. The queen mother, whose dominance over her son made her the true ruler of France, attempted to maintain a balance of power between the two factions so that neither would be strong enough to challenge her own position.

Showing favor to the Huguenots, in 1572 Catherine arranged a marriage between her daughter Margaret and Henry of Navarre (later Henry IV), a Huguenot. However, Catherine soon grew to resent the influence that the Huguenot leader Coligny was gaining over her son. She conspired with the Guises in a plot to murder Coligny. When the plot failed, the queen, fearful that her part in it would be revealed, told the king that the Huguenots planned to assassinate him. After hours of argument, she won his consent to the slaying of the Huguenot leaders gathered in Paris for the wedding of Margaret and Henry of Navarre.

Early on the morning of St. Bartholomew's Day (August 24), assassins murdered Coligny and most of the other leading Huguenots. Henry of Navarre was spared, but was forced to renounce his religion. (The renunciation was insincere, and Henry remained a Protestant until 1593.) Paris mobs, stirred by the leaders of the plot, slaughtered thousands of Huguenot men, women, and children. Similar massacres occurred in many other parts of France. No reliable figure is known for the total number of victims, but some authorities estimate it to have been at least 10,000.