A justacorps or justaucorps () is a knee-length coat worn by men in the latter half of the 17th century and throughout the 18th century. It is of French origin, where it had developed from a cape-like garment called a casaque. It was introduced into England as a component of a three-piece ensemble, which also included breeches and a long vest or waistcoat. This ensemble served as the prototype for the modern-day three-piece suit. The justacorps itself evolved into the frock coat.
The fabric selection and styling of the justacorps varied over time, as fashions altered through history. Elaborated forms featuring rich embroidery and embellishments were influenced by Indo-Persian and Turkish garments, which cultures influential Europeans were increasingly in contact with, through travel, trade, and diplomatic missions. East European garments of similar cut became fashionable around the same time as the justacorp arrived in Western Europe, and its construction and decoration were also influenced by "oriental" modes.