Catherine Monvoisin, or Montvoisin, née Deshayes, known as "La Voisin", was a French fortune teller, commissioned poisoner, and professional provider of alleged sorcery. She was the head of a network of fortune tellers in Paris providing poison, aphrodisiacs, abortion, purported magical services and the arranging of black masses, with clients among the aristocracy and became the central figure in the famous affaire des poisons. Her purported organization of commissioned black magic and poison murder was suspected to have killed 1,000 people, but it is believed that upwards of 2,500 people might have been murdered.

The Affair of the Poisons was a major murder scandal in France during the reign of King Louis XIV. Between 1677 and 1682, a number of prominent members of the aristocracy were implicated and sentenced on charges of poisoning and witchcraft. The scandal reached into the inner circle of the king. It led to the execution of 36 people.