Sienna is a form of limonite clay used in the production of oil paint pigments. Its yellow-brown colour comes from ferric oxides contained within. As a natural pigment, it (along with its chemical cousins ochre and umber) was one of the first pigments to be used by humans, and is found in many cave paintings. When roasted, the pigment becomes a rich reddish-brown, known as burnt sienna. The first recorded use of sienna as a colour name in English was in 1760.

Sienna refers to a wide range of colors, from a dark reddish brown (burnt sienna) to a medium yellowish brown (raw sienna). The color meant by sienna varies according to the color list and the country.

The name derives from the most notable Renaissance location for the earth, Siena, Italy, and is short for terra di Siena, "earth of Siena." The mines used to produce this sienna petered out in the 1940s. Much of today's sienna production is still carried out in the Italian islands of Sardinia and Sicily, while other major deposits are found in the Appalachian Mountains, where it often goes hand-in-hand with the region's iron deposits.