Gilles de Rais (date of birth unknown, not earlier than 1405 – 26 October 1440), Baron de Rais (French: [də ʁɛ]), was a knight and lord from Brittany, Anjou and Poitou, a leader in the French army, and a companion-in-arms of Joan of Arc. He is best known for his reputation and later conviction as a confessed serial killer of children.

A member of the House of Montmorency-Laval, Gilles de Rais grew up under the tutelage of his maternal grandfather and increased his fortune by marriage. He earned the favour of the Duke of Brittany and was admitted to the French court. From 1427 to 1435, Rais served as a commander in the French army, and fought alongside Joan of Arc against the English and their Burgundian allies during the Hundred Years' War, for which he was appointed Marshal of France.

In 1434 or 1435, he retired from military life, depleted his wealth by staging an extravagant theatrical spectacle of his own composition, and was accused of dabbling in the occult. After 1432, Rais was accused of engaging in a series of child murders, with victims possibly numbering in the hundreds. The killings came to an end in 1440, when a violent dispute with a clergyman led to an ecclesiastical investigation that brought the crimes to light, and attributed them to Rais. At his trial, the parents of missing children in the surrounding area and Rais's own confederates in crime testified against him. He was condemned to death and hanged at Nantes on 26 October 1440.

Rais is believed to be the inspiration for the French folktale "Bluebeard" ("Barbe bleue"), which is earliest recorded in 1697.