A punching bag is a sturdy bag designed to be repeatedly punched, for use in exercise or physical training.

Someone who trains using a punching bag is usually trying to improve one of three areas: physical strength, cardiac fitness , or technique.

Punching bags are often filled with grains or other material, and are usually affixed to, or hung from, a stand of some sort. The design of a punching bag should allow it to take repeated and constant physical abuse and not break. It must also absorb the impact of blows, without causing undue harm to the user.

Punching bags of one sort or another have probably existed from antiquity. They are used most notably in boxing training and martial arts training.

Metaphorically, a person is called a "punching bag" if one perceives the person as being the subject of constant and unwarranted attacks.