Brassica nigra (black mustard) is an annual weedy plant cultivated for its seeds, which are commonly used as a spice. The plant is believed to be native to the southern Mediterranean region of Europe, and has been cultivated for thousands of years.

The spice is generally made from ground seeds of the plant, with the seed coats removed. The small (1 mm) seeds are hard and vary in color from dark brown to black. They are very flavorful, although they have almost no aroma. The seeds are commonly used in Indian cuisine, for example in curry. The seeds are usually thrown into hot oil or ghee after which they pop, releasing a characteristic 'nutty' flavor. The seeds have a significant amount of fatty oil. This oil is used often as cooking oil in India.

Ground seeds of the plant mixed with honey are widely used in eastern Europe as cough suppressant. In Eastern Canada, the use of "mouche de moutarde" to treat respiratory infections was popular before the advent of modern medicine. It consisted in mixing ground mustard seeds with flour and water, and creating a cataplasm with the paste. This cataplasm was put on the chest or the back and left until the person felt a stinging sensation. The plant itself can grow from 2 to 8 feet tall with racemes of small yellow flowers. These flowers are usually up to 1/3" across, with 4 petals each. Its leaves are covered in small hairs. The leaves can wilt on hot days, but recover at night.

Since the 1950s, black mustard has become less popular as compared to brown mustard because some cultivars of brown mustard have seeds that can be mechanically harvested in a more efficient manner.

It is theorized that black mustard is the seed mentioned by Jesus in .