Plaster of Paris, or simply plaster, is a type of building material based on calcium sulfate hemihydrate, nominally (CaSO4)2. H2O. It is created by heating gypsum to about 150C, 2(CaSO4.2 H2O) => (CaSO4)2 .H2O + 3 H2O (released as steam). A large gypsum deposit at Montmartre in Paris is the source of the name. When the dry plaster power is mixed with water, it re-forms into gypsum, initially as a paste but eventually drying into a solid. The structure is made up of sheets of Ca2+ and SO42- ions held together by hydrogen bonds in the water molecules. The grip between these sheets is easily broken, so plaster is fairly soft.

Plaster is used as a building material similar to mortar or cement. Like those materials plaster starts as a dry powder that is mixed with water to form a paste, which then dries into a hard surface. Unlike those materials plaster remains quite soft after drying, and can be easily manupulated with metal tools or even sandpaper. Plaster was a common building material for wall surfaces in a process known as lath and plaster, in which a series of wooden strips were covered with a semi-dry plaster and then hardened into a flat surface. Today this building method has been almost completely replaced with drywall.

After mixing plaster expands while drying, then contracts slightly just before hardening completely. This makes plaster excellent for use in molds, and it is often used as an artistic material for casting. Plaster is also commonly spread over an armature (form), usually made of wire, mesh or other materials. In medicine, it is also widely used as a support for broken bones; a bandage impregnated with plaster is moistened and then wrapped around the damaged limb, setting into a close-fitting yet easily removed tube.

Many early buildings in Paris, Ontario were made with abundant amounts of this material, hence the town's name.