Boiled leather, often referred to by its French translation, cuir bouilli (French: [kɥiʁ buji]), was a historical material for various uses common in the Middle Ages and Early Modern Period. 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 actual boiling of the leather was not part of the process, but immersion in water, cold or hot, was.