Leonardo di ser Piero da Vinci (1452 - 1519)
La belle ferronnière
Oil on wood, 1490-1496
62 cm × 44 cm (24 in × 17 in)
Musée du Louvre, Paris, France

La belle ferronnière is a name that has been applied to two Renaissance portrait paintings. The first (illustrated), though sometimes simply known as Portrait of an Unknown Woman, may be of Lucrezia Crivelli, a mistress of Ludovico Sforza, and is attributed by the Musée du Louvre, it is conserved, to the school of Leonardo da Vinci in Milan; The other portrait, likely to be of Cecilia Gallerani, another of Ludovico's mistresses, and which is almost certainly by da Vinci, is more often called Lady with an Ermine.

The painting's title, applied as early as the seventeenth century, identifying the sitter as the wife or daughter of an ironmonger (a ferronnier), was said to be discreetly alluding to a reputed mistress of François I married to a certain Le Ferron— itself a Romantic legend of revenge, in which the aggrieved husband intentionally infected himself with syphilis, which he passed to the king through infecting his wife— which was applied to Leonardo's Lady with an Ermine when it was in Princess Czartoryski's collection, became confused with this portrait by the presence in this image also, of a jewel worn on a delicate chain across the forehead.

Bernard Berenson attributed this Leonardesque portrait to Bernardino de' Conti. Giovanni Antonio Boltraffio was suggested by Herbert Cook, who retracted his opinion, seeing Leonardo's own hand, in 1904.В 

The painting was briefly featured in the opening sequence of the 2006 film The Da Vinci Code.

A later version of the painting had been about to be sold to the Kansas City Art Institute as the original, but was identified as a copy by Sir Joseph Duveen, who permitted his remarks to be published in the New York World in 1920; the owner, Mrs Andrée Lardoux Hahn, sued for defamation of property in a notorious court case, which involved many of the major connoisseurs of the day, inspecting the two paintings side by side at the Louvre; the case was eventually heard in New York before a jury selected for not knowing anything of Leonardo or Morellian connoisseurship, and settled for $60,000 plus court expenses, which were considerable. The owner's account, Harry Hahn's The Rape of La Belle (1946) is a classic of conspiracy theory applied to the art world.

A nineteenth-century copy of La Belle Ferronnière is conserved in the Musée des beaux-arts, Chambéry.