Boiled leather, often referred to by its French translation, cuir bouilli (French: [kɥiʁ buji]), was a historical material common in the Middle Ages and Early Modern Period and used for various purposes. It was leather that had been treated so that it became tough and rigid, as well as able to hold moulded decoration. It was the usual material for the robust carrying-cases that were made for important pieces of metalwork, instruments such as astrolabes, personal sets of cutlery, books, pens and the like. It was used for some armour, being both much cheaper and much lighter than plate armour, but could not withstand a direct blow from a blade, nor a gunshot.

Alternative names are "moulded leather" and "hardened leather". In the course of making the material it becomes very soft, and can be impressed into a mould to give it the desired shape and decoration, which most surviving examples have. Pieces such as chests and coffers also usually have a wooden inner core.

Various recipes for making cuir bouilli survive, and do not agree with each other; probably there were a range of recipes, partly reflecting different final uses. Vegetable-tanned leather is generally specified. Scholars have debated the subject at length and attempted to recreate the historical material. Many, but not all, sources agree that the process involved immersion of the leather in water, but not actual boiling.

Cuir bouilli was used for cheap and light armour, although it was much less effective than plate armour, which was extremely expensive and too heavy for much to be worn by infantry (as opposed to knights fighting on horseback). However, cuir bouilli could be reinforced against slashing blows by the addition of metal bands or strips, especially in helmets. Modern experiments on simple cuir bouilli have shown that it can reduce the depth of an arrow wound considerably, especially if coated with a crushed mineral facing mixed with glue, as one medieval Arab author recommended.

In addition, "armour based on hide has the unique advantage that it can, in extremis, provide some nutrition" when actually boiled. Josephus records that the Jewish defenders in the Siege of Jerusalem in AD 70 were reduced to eating their shields and other leather kit, as was the Spanish expedition of Tristan de Luna in 1559.

Versions of cuir bouilli were used since ancient times, especially for shields, in many parts of the world. Although in general leather does not survive long burial, and excavated archaeological evidence for it is rare, an Irish shield of cuir bouilli with wooden formers, deposited in a peat bog, has survived for some 2,500 years. It was commonly used in the Western world for helmets; the pickelhaube, the standard German helmet, was not replaced by a steel stahlhelm until 1916, in the midst of World War I. As leather does not conduct heat the way metal does, firemen continued to use boiled leather helmets until World War II, and the invention of strong plastics.

The word cuirass for a breastplate indicates that these were originally made of leather. In the Late Middle Ages, the heyday of plate armour, cuir bouilli continued to be used even by the rich for horse armour and often for tournament armour, as well as by ordinary infantry soldiers. Tournaments were increasingly regulated in order to reduce the risk to life, and in 1278 Edward I of England organized one in Windsor Great Park at which cuir bouilli armour was worn, and the king provided swords made of whale bone and parchment.

The account of the Battle of Agincourt in 1415 by Jean de Wavrin, who was present on the French side, describes the crucial force of English longbowmen as having on their heads either cuir bouilli helmets, or wicker with iron strips, or nothing (the last, he says, were also barefoot).