Catherine Monvoisin, or Montvoisin, née Deshayes, known as "La Voisin" (c. 1640 – 22 February 1680), was a French fortune teller, commissioned poisoner, and professional provider of alleged sorcery.

She learned fortune telling as a child and later married Antoine Monvoisin, who was active as a jeweller and silk merchant with a shop at Pont-Marie in Paris.

[2] When her husband's trade business led to bankruptcy, La Voisin supported the family by practising chiromancy and face-reading.

She resided at Villeneuve-sur-Gravois, where she received her clients all day, and entertained the Parisian upper-class society at parties with violin music in her garden at night.

[2] At one point, Adam Lesage tried to induce her to kill her husband, but while he was initially successful, she changed her mind and aborted the process.

[2] La Voisin was interested in science and alchemy and financed several private projects and enterprises, some of them concocted by con artists who tried to swindle her.

[2] She was known to suffer from alcoholism, was apparently abused by Latour, and engaged in several conflicts with her rival, the poisoner Marie Bosse.

[2] She spent a great deal of money in order to provide an atmosphere which would make her clients more inclined to believe her prophecies: for example, she had a special robe of crimson red velvet embroidered with eagles in gold made for a price of 1,500 livres to perform in.

These actions were initially to visit the church of some particular saint; then she started to sell amulets, and gradually, she recommended more and more alleged magical objects or rituals of various kinds.

[2] The most important client of La Voisin was Madame de Montespan, the official royal mistress to King Louis XIV of France.

During the King's affair with Madame de Soubise, Montespan used an aphrodisiac provided by Voisin's colleague Françoise Filastre and made by Louis Galet in Normandy.

[2] The death of the King's sister-in-law, the duchesse d'Orléans, had been falsely attributed to poison, and the crimes of Madame de Brinvilliers (executed in 1676) and her accomplices were still fresh in the public mind.

In 1677, the fortune teller Magdelaine de La Grange was arrested for poisoning, and claimed that she had information about crimes of high importance.

On 12 March 1679, La Voisin was arrested outside Notre-Dame de Bonne-Nouvelle after having heard mass, just before her meeting with Catherine Trianon.

In March, however, she named Marguerite Leféron and Francoise de Dreux as clients, and on 10 October, she admitted having sold poison and magical services to several members of the royal court; she also described the development of her career.

[2] Her list of clients, the arranging of the black masses, her connection to Montespan and the murder attempt on the King were not revealed until after her death, when it was stated by her daughter and confirmed by the testimonies of her former associates.

Another accusation she denied includes injecting syringes with fatal liquids into the bodies of pregnant women to "empty" them, after which the aborted foetuses were subsequently buried in the same manner at the garden grave.

This caused the monarch to eventually close the investigation, seal the testimonies and place the remaining accused outside of the public justice system by imprisoning them under a lettre de cachet.

Belgian black metal band Enthroned use La Voisin as the basis for the song "Graced by Evil Blood" on their 2002 album Carnage in Worlds Beyond.