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

In the New Testament, the mustard seed is used by Jesus in the parable of the Mustard Seed as a model for the kingdom of God which initially starts small but grows to be the biggest of all garden plants. Faith is also spoken about in the context of a mustard seed.

The earliest reference to mustard is in India from a story of Gautama Buddha in the 5th century BCE. Gautama Buddha told the story of the grieving mother (Kisa Gotami) and the mustard seed. When a mother loses her only son, she takes his body to the Buddha to find a cure. The Buddha asks her to bring a handful of mustard seeds from a family that has never lost a child, husband, parent or friend. When the mother is unable to find such a house in her village, she realizes that death is common to all, and she cannot be selfish in her grief.The Buddha stated that if an individual were to pick a single mustard seed every hundred years from a seven-mile cube worth of mustard seeds, then by the time the last seed is picked, the age of the world cycle would still continue. (If a mustard seed is 3 mm in diameter, then taking one seed every 100 years from a seven-mile cube of seeds, would take 936 quintillion years, 68 billion times the age of the universe.)

Jewish texts compare the knowable universe to the size of a mustard seed to demonstrate the world's insignificance and to teach humilityThe Jewish philosopher Nahmanidesmentions the universe expanded from the time of its creation, in which it was the size of a mustard seed.