The 88 mm gun (eighty-eight) was a German anti-aircraft and anti-tank artillery gun from World War II. It was widely used by Germany throughout the war, and was one of the most recognized German weapons of the war. Development of the original models led to a wide variety of guns.

The name applies to a series of guns, the first one officially called the 8,8 cm Flak 18, the improved 8,8 cm Flak 36, and later the 8,8 cm Flak 37. Flak is a contraction of German Flugzeugabwehrkanone meaning "anti-aircraft cannon", the original purpose of the eighty-eight. In informal German use, the guns were universally known as the Acht-acht ("eight-eight"), a contraction of Acht-komma-acht Zentimeter ("8.8 cm"). In English, "flak" became a generic term for ground anti-aircraft fire.

The versatile carriage allowed the eighty-eight to be fired in a limited anti-tank mode when still on wheels, and to be completely emplaced in only two-and-a-half minutes. Its successful use as an improvised anti-tank gun led to the development of a tank gun based upon it. These related guns served as the main armament of tanks such as the Tiger I: the 8.8cm KwK 36, with the "KwK" abbreviation standing for KampfwagenKanone ("fighting vehicle cannon").

In addition to these Krupp's designs, Rheinmetall created later a more powerful anti-aircraft gun, the 8,8 cm Flak 41, produced in relatively small numbers. Krupp responded with another prototype of the long-barreled 88 mm gun, which was further developed into the anti-tank and tank destroyer 8.8cm Pak 43 gun, and turret-mounted 8.8cm KwK 43 heavy tank gun.

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