The Affair of the Poisons (French: affaire des poisons, pronounced [afɛʁ de pwazɔ̃]) 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.

In 1672, upon the natural death of cavalry officer Godin de Sainte-Croix, an inventory of his papers revealed a box containing nine letters from his mistress, the Marquise de Brinvilliers, as well as the marquise's acknowledgment of a debt amounting to 30,000 livres. This box also contained various vials which, after being analyzed by an apothecary, were found to have contained various poisons that left little trace in the body.

The case began in 1675 after the trial of de Brinvilliers, who was accused of having conspired with de Sainte-Croix to poison her father Antoine Dreux d'Aubray in 1666, and two of her brothers, Antoine d'Aubray and François d'Aubray, in 1670, in order to inherit their estates. There were also rumours that she had poisoned poor people during her visits to hospitals.

After being accused, she fled but was arrested in Liège. De Brinvilliers was tortured and confessed, was sentenced to death, and on 17 July was tortured with the water cure (forced to drink sixteen pints of water) and then beheaded, and her body burned at the stake. Her alleged accomplice Sainte-Croix did not face charges because he had died of natural causes in 1672.

The sensational trial drew attention to other mysterious deaths, starting rumours. Prominent people, including Louis XIV, became alarmed that they might be poisoned.

The affair proper opened in February 1677 after the arrest of Magdelaine de La Grange on charges of forgery and murder. La Grange appealed to François Michel Le Tellier, Marquis of Louvois, claiming that she had information about other crimes of high importance. Louvois reported to the king, who told Gabriel Nicolas de la Reynie, who, among other things, was the chief of the Paris police, to root out the poisoners. La Reynie sought to calm the king. The subsequent investigation of potential poisoners led to accusations of witchcraft, murder and more.

Authorities rounded up a number of fortune tellers and alchemists who were suspected of selling not only divinations, séances and aphrodisiacs, but also "inheritance powders" (a euphemism for poison). Some of them confessed under torture and gave authorities lists of their clients, who had allegedly bought poison to get rid of their spouses or rivals in the royal court.

The most famous case was that of the midwife Catherine Deshayes Monvoisin or La Voisin, who was arrested in 1679 after she was incriminated by the poisoner Marie Bosse. La Voisin implicated several important courtiers. These included Olympia Mancini, the Countess of Soissons, her sister, the Duchess of Bouillon, François Henri de Montmorency, Duke of Luxembourg and, most importantly, the king's mistress, Madame de Montespan.