A justacorps or justaucorps is a long, knee-length coat worn by men in the latter half of the 17th century, and throughout the 18th century. The garment is of French origin, and was introduced in England as a component of a three-piece ensemble, also consisting of breeches and a long vest or waistcoat. This ensemble served as the prototype of the modern-day three-piece suit. The fabric selection and styling of the justacorps varied throughout time periods, as fashions frequently altered.

In 1666, Charles II of England declared to reset men’s fashion by introducing a new garment, referred to as a vest, or a waistcoat. The vest was knee-length, worn in conjunction with breeches and an overcoat of equal length. The coat became known as the justacorps, and this outfit is thought to be the prototype of the modern day men’s three-piece suit.