Mustard seeds are the small round seeds of various mustard plants. The seeds are usually about 1 to 2 millimetres (0.039 to 0.079 in) in diameter and may be colored from yellowish white to black. They are an important spice in many regional foods and may come from one of three different plants: black mustard (Brassica nigra), brown Indian mustard (B. juncea), or white/yellow mustard (B. hirta/Sinapis alba).

Mustard seeds generally take eight to ten days to germinate if placed under the proper conditions, which include a cold atmosphere and relatively moist soil. Mature mustard plants grow into shrubs.

Mustard grows well in temperate regions. Major producers of mustard seeds include India, Pakistan, Canada, Nepal, Hungary, Great Britain and the United States. Brown and black mustard seeds return higher yields than their yellow counterparts.