Tadelakt (Moroccan Arabic: تدلاكت, romanized: tadlākt) is a waterproof plaster surface used in Moroccan architecture to make baths, sinks, water vessels, interior and exterior walls, ceilings, roofs, and floors. It is made from lime plaster, which is rammed, polished, and treated with soap to make it waterproof and water-repellent. Tadelakt is labour-intensive to install, but durable. Since it is applied as a paste, tadelakt has a soft, undulating character, it can form curves, and it is seamless. Pigment can be added to give it any colour, but deep red is traditional. It may have a shiny or matte finish.

The term tadelakt, meaning "to rub in", is an Amazighified expression from the Arabic word ‏تدليك‎ tadlīk, meaning "to rub or massage."

Tadelakt is thought to have evolved from qadad, a similar plaster used in Yemen for millennia that is treated with calcium hydroxide and oils and fats instead of soaps.

The basic constituents of tadelakt plaster are:

The soap chemically reacts with the lime plaster, forming lime (calcium) soaps. Calcium soaps are insoluble in water, and fairly hard. They are familiar, in areas with calcium-rich ("hard") water, as deposits in bathtubs, sinks, and showers; when soap is mixed with the water's dissolved calcium carbonate/lime, calcium soaps form.

Traditional application includes polishing with a river stone and treatment with oleic acid, in the form of olive oil soap, to lend it its final appearance and water resistance.

In Morocco, the traditional application technique:

Long-term maintenance of tadelakt requires regularly re-sealing the surface with a soap solution; in the case of qadad roofs, this was traditionally done every few years.