Isatis tinctoria, also called woad (), dyer's woad, dyer's-weed, or glastum, is a flowering plant in the family Brassicaceae (the mustard family) with a documented history of use as a blue dye and medicinal plant. The double use of woad is seen in its name: the term Isatis is linked to its ancient use to treat wounds; the term tinctoria references its use as a dye. It is occasionally known as Asp of Jerusalem. Woad is also the name of a blue dye produced from the leaves of the plant. Woad is native to the steppe and desert zones of the Caucasus, Central Asia to Eastern Siberia and Western Asia but is now also found in South-Eastern and Central Europe and western North America.

Woad was an important source of blue dye and was cultivated throughout Europe, especially in Western and Southern Europe. In medieval times, there were important woad-growing regions in England, Germany and France. Towns such as Toulouse became prosperous from the woad trade. Over time, woad's cultivation spread across Europe, where it was an important trade good. From the 16th century, indigo (Indigofera tinctoria), a plant from the New World, started to replace it. In the early 20th century, both woad and Indigofera tinctoria were mostly replaced by synthetic blue dyes, though there has also been some revival of the use of woad for craft purposes.

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