A kinetic energy penetrator is a type of ammunition which uses kinetic energy as the primary means of penetrating armour. Ammunition of this kind lacks explosive, so in order to successfully penetrate modern armour, the round must be fired with a very high muzzle velocity, and the force of impact must be concentrated on a small area. These requirements have led to the round commonly being shaped as a long sleek rod.

To produce very high speeds the ammunition is normally composed of a narrow penetrator surrounded by a sabot which expands the diameter to the full barrel width of the firing gun. This allows the pressure of the propellant gasses to act on the full-size base and produce rapid acceleration of the round, which is lighter than a full metal round of the same diameter would be. Once the round leaves the barrel the sabot falls off, leaving the penetrator travelling at high speed and with a smaller cross-sectional area, which reduces aerodynamic drag during the flight to the target. This technique was first used in anti-tank guns during World War 2.

KE-penetrators for tanks are commonly just 2-3 centimeters in diameter, and 50-60 centimeters long. To maximize the amount of kinetic energy released on the target, the penetrator must be made of a heavy material, such as tungsten or DU (depleted uranium). The hardness of the penetrator is of lesser importance. In fact uranium is not particularly hard. An advantage of uranium is that it is pyrophoric: the molten fragments of the penetrator ignite on contact with air. Uranium rod is also self-sharpening on impact, but few countries use it, because DU is a public relations disaster.

Because a long, thin rod is aerodynamically unstable and tends to tumble in flight, two different approaches have been used to stabilise them. The oldest is rifling, which spins the round. An alternative approach is to add fins like those of an arrow to the base and fire the round from a smooth-bore gun. This is the approach commonly used in recent Russian, German and US guns. Sometimes a rifled barrel has been combined with fin stabilisation, using some system to prevent the round from spinning in the barrel. The rifled barrel approach improves the accuracy of the other types of ammunition which must be fired. An example of this is the L30 120 mm rifled tank gun used on the United Kingdom Challenger 2 MBT.

Other names for KE-penetrators include APFSDS (Armour Piercing Fin Stabilized Discarding Sabot) and long-rod penetrator. It is generally accepted that KE-penetrators are the most effective ammunition in penetrating armour today. The other main type of tank ammunition is High explosive anti-tank (more common in missiles). The British prefer HESH (High Explosive Squash Head).

A typical rifle bullet is also a kinetic energy penetrator when faced with either body armour or lightly armoured vehicles and there have at times been moves to use the sabot and thin penetrator approach in rifle ammunition, either to lower the weight of the round or to increase penetration. These attempts have generally been less successful than conventional rounds for other target types and have not replaced them as general purpose ammunition, partially because of restrictions imposed by the Geneva Conventions.