An assault gun (from German: Sturmgeschütz, lit. 'storm gun', meaning "attack gun") is a type of armored infantry support vehicle and self-propelled artillery, mounting an infantry support gun on a protected self-propelled chassis, intended for providing infantry with heavy direct fire support during engagement, especially against other infantry or fortified positions, secondarily also giving some armored protection and anti-armor capability.

Assault guns were pioneered by the Soviet Union and Nazi Germany during the 1930s, initially being self-propelled guns with direct fire in mind (such as the Soviet SU-5-1), with Germany introducing the first purpose-built (and purpose-named) assault gun, the Sturmgeschütz III, in 1940.

The core concept of the assault gun is a protected self propelled infantry gun. This means it is intended to be used by infantry brigades in order to give infantry mobile heavy direct fire capability against protected enemy positions and threats, to be able move with infantry in assaults, and secondarily to give some armored protection against enemy fire.

Historically, the concept of assault guns was very similar to that of the infantry tank, as both were combat vehicles intended to accompany infantry formations into battle, but where assault gun designs often skipped tank features and design elements deemed unnecessary for reasons of cost and doctrine. However, during World War II assault guns were more mobile than tanks and could be utilized as both direct and indirect fire artillery. Although they could approximate the firepower of a tank, assault guns mostly fired high explosive shells at relatively low velocities, which were well suited for their role of knocking out hard points such as fortified positions and buildings. They were not intended to be deployed as tank substitutes or dedicated tank destroyers. Nevertheless, as the conflict progressed, the increasing proliferation of tanks on the battlefield forced many assault gun units to engage armor in defense of the infantry, and led to armies becoming more dependent on multipurpose designs which combined the traditionally separate roles of an assault gun and a tank destroyer.

German and Soviet assault guns introduced during World War II usually carried their main armament in a fully enclosed casemate rather than a gun turret. Although this limited the field of fire and traverse of the armament, it also had the advantage of a reduced silhouette and simplified the manufacturing process. The United States never developed a purpose-built assault gun during the war, although it did modify preexisting armored fighting vehicles for that role, including the M4 Sherman (as the M4(105)), the M5 Stuart (as the M8 Scott), and the M3 half-track (as the T19 Howitzer Motor Carriage). The classic assault gun concept was largely abandoned during the postwar era in favor of tanks or multipurpose tank destroyers attached to infantry formations, which were also capable of providing direct fire support as needed. In the United States and most Western countries, the assault gun ceased to be recognized as a unique niche, with individual examples being classified either as a self-propelled howitzer or a tank, one exception being Sweden, which continued to develop casemate assault guns post-war, such as the Infanterikanonvagn 72, all the way into the 1960s before settling on a turreted design in 1968, becoming the Infanterikanonvagn 91. The Soviet Union continued funding development of new assault guns as late as 1967, although few of its postwar designs were adopted in large numbers. In Soviet and Eastern European armies, the traditional assault gun was primarily superseded by tank destroyers, such as the SU-100, which is capable of supporting either infantry or armor. Since the 1980s, the multi-purpose assault gun concept has seen a resurgence, mainly in the form of turreted wheeled designs, such as the South African Rooikat and Italian B1 Centauro. Today, modern assault guns include the Japanese Type 16 maneuver combat vehicle and the American M1128 Stryker and M10 Booker.

Assault guns were primarily developed during World War II by the forces of Nazi Germany and the Soviet Union. Early in the war, the Germans began to create makeshift assault guns by mounting their infantry support weapons on the bed of a truck or on obsolete tanks with the turret removed. Later in the war, both the Germans and the Soviets introduced fully armoured purpose-built assault guns into their arsenals.

Early on, the Soviets built the KV-2, a variant of the KV-1 heavy tank with a short-barreled 152 mm howitzer mounted in an oversized turret. This was not a success in battle, and was replaced with a very successful series of turretless assault guns: the SU-76, SU-122, and the heavy SU-152, which were followed by the ISU-122 and ISU-152 on the new IS heavy tank chassis.

The primary German assault gun was the Sturmgeschütz III (StuG III). At about the same time (March 1942) as the howitzer-like KwK 37 gun was dropped from the Panzer IV's use, its Sturmkanone equivalent in the StuG III up to that time, was likewise replaced with a longer-barreled, high-velocity dual-purpose 75 mm gun that had also been derived from the successful PaK 40 anti-tank towed artillery piece. The Germans also built a number of other fully armoured turretless assault guns, including the StuG IV, StuIG 33B, Brummbär and Sturmtiger. This last one was a very heavy vehicle, and was built only in small quantities.