Charles IX, 1550-74, king of France. He succeeded (1560) his brother Francis II under the regency of his mother, Catherine de' Medici. She retained her influence throughout his reign. After 1570, however, Charles was temporarily under the sway of the French Huguenot leader Gaspard de Coligny. Catherine, fearing for her power, persuaded her weak son to approve the massacre of Saint Bartholomew's Day in which Coligny and thousands of other Huguenots were murdered. Charles IX was succeeded by his brother Henry III.