A hair shirt is a coarse garment which intended to be worn next to the skin, thus keeping the wearer in a state of discomfort and constant awareness of the shirt's presence. Such garments were traditionally worn by some Christian religious orders, along with individuals who felt penitent about certain actions or their lifestyles. The use of hair shirts is fairly limited in the modern era, but the term is often used metaphorically, which is why someone might refer to wearing a hair shirt when they perform some other act of self-imposed penitence.

Originally, hair shirts were known as cilices, in a reference to the Latin word cilicium, meaning “covering made from goat's hair.” Early hair shirts were made from sackcloth or coarse animal hair so that they irritated the skin, and later versions integrated additional uncomfortable features such as thin wires or twigs. Several characters in the Bible wore hair shirts as demonstrations of religious faith, and the practice was picked up by devout members of society and the Church. The term “cilice” is now used more generally for any object worn to increase discomfort.

The tradition of wearing hair shirts is part of a practice called the mortification of the flesh. Mortification ranges from a simple denial of pleasures to actually committing serious injury, as in the case of medieval flagellants who whipped themselves during the time of the Plague. Mortification is said to be an expression of faith, and it is also meant to train the soul, expelling sin and promoting pious, faithful behavior. Extreme mortification is no longer in vogue in most Christian sects, but more mild forms like fasting are an important part of religious faith for some devout Christians.

Religious ascetics often engage in mortification to bring themselves closer to God, and to keep their own religious practice humble. In the era when the hair shirt was a popular form of mortification, many others in society ranging from kings to merchants wore hair shirts. Members of the upper class engaged in mortification to encourage themselves to be mindful of God, and sometimes to do penance for decking themselves out in fine clothing. Hair shirts were usually worn under other garments, because showing a hair shirt off would be a form of vanity, punishable by additional penances.

While the concept of a hair shirt may seem archaic or repugnant, awareness of suffering is an important aspect of many religions, from Buddhism to traditional shamanistic practices. Since hair shirts are worn as a form of voluntary mortification, they should not be considered in the same category with acts of torture and forced penance.