A stirrup is a light frame or ring that holds the foot of a rider, attached to the saddle by a strap, often called a stirrup leather. Stirrups are usually paired and are used to aid in mounting and as a support while using a riding animal (usually a horse or other equine, such as a mule). They greatly increase the rider's ability to stay in the saddle and control the mount, increasing the animal's usefulness to humans in areas such as communication, transportation and warfare.

In antiquity, the earliest foot supports consisted of riders placing their feet under a girth or using a simple toe loop. Later, a single stirrup was used as a mounting aid, and paired stirrups appeared after the invention of the treed saddle. The use of paired stirrups is credited to the Chinese Jin Dynasty and came to Europe during the Middle Ages. Some argue that the stirrup was one of the basic tools used to create and spread modern civilization, possibly as important as the wheel or printing press.

Modern stirrups come in a wide variety of styles, sizes and materials and are attached to most saddles by means of adjustable stirrup leathers, which can be altered in length to fit both the size of the rider and the need to remain over the horse's optimal center of balance for a given equestrianism discipline. There are safety concerns associated with the use of stirrups, including a risk that a fallen rider may get their foot caught in the stirrup and be dragged by the horse, or that long hours of use without rest may cause problems in the human foot's Peroneus Tertius tendon. Stirrups are safer to use when riding boots are worn, and proper sizing and placement of the foot on the stirrup increases both safety and usability.