An undershirt in American English (vest in British and South African English, banyan in the Indian Subcontinent, or singlet in Australia and New Zealand), is an article of underwear worn underneath a dress shirt (or sometimes T-shirt) intended to protect it from body sweat and odors. It can have short sleeves (T-shirt) or be sleeveless (A-shirt). The term most commonly refers to upper-body innerwear worn by males.

An undershirt may be worn to protect the body from stiff or otherwise uncomfortable fabric. It also makes dress shirts less transparent, to reduce the amount of sweat absorbed by the shirt's fabric. It can be worn during winter months as an extra layer of warmth, and reduces wear on the upper layers of clothing.