Catherine de Medici is the overarching antagonist of the Doctor Who story "The Massacre of St Bartholomew's Eve".
She was portrayed by the late Joan Young.
Queen Mother of France and mother of King Charles IX, Catherine de Medici was one of the country's leading Catholics and opposed the King's lenient policies towards the Protestant Huguenots. The marriage of her daughter to the Protestant King Henry of Navarre, and the growing influence on the King of Admiral de Coligny who openly challenged her in council, convinced her to conspire with Marshall Tavannes to attempt to have de Coligny assassinated.
When the attempt failed, partly thanks to the intervention of the Doctor's friend Steven Taylor, and the King ordered an investigation, Catherine met him in private and openly admitted that she and Tavannes had hired the assassin. She convinced Charles that, with Henry of Navarre as an alternative Protestant claimant, the Huguenots were now a threat to him. Although Tavannes had made plans to target leading Protestants in precise assassinations, Catherine convinced the King to instead give the order for a mob to be stirred up against all Huguenots, insisting that no "heretic" was innocent. Tavannes convinced her that allowing Henry to be killed with the others would mean war and Catherine reluctantly agreed that Tavannes should get him to safety before the massacre started. She then ordered him to have the city sealed.