Armyworm, the caterpillar of any of several species of owlet moth. Armyworms are one to two inches (2.5 to 5 cm) in length and have sparse hair or are hairless. They are typically greenish or brownish, with yellow stripes. They are active mostly at night. North American species include the common armyworm, beet armyworm, and fall armyworm.

Armyworms derive their name from the fact that masses of them often move together in search of food, thus resembling an army on the march. Armyworms feed on the foliage of many kinds of crops, including corn, peas, beets, tomatoes, and wheat. The bacterium Bacillus thuringiensis is used to infect and kill armyworms.

Armyworms belong to the family Noctuidae. The common armyworm is Pseudaletia unipuncta; the beet armyworm, Spodoptera exigua; the fall armyworm, S. frugiperda.