A Ka-bar is the 7-inch fighting and utility knife used by the US Marines in World War II, and has been carried into battle by generations of Marines since. The knife itself is a Bowie knife with a seven-inch (17cm) blade, and was originally designed as a hunting knife around 1890 and would have been considered unremarkable in 1820. When war broke out, the US Marines anticipated a need for knives and daggers suited to trench warfare, and someone chose the Ka-Bar design from a catalog of hunting gear (the Marines bought a great many knives of a great many different designs from a great many suppliers during the war, but it was the Ka-Bar that was most common, most popular, and chosen for continued purchase and issue after the war was over). The sole change made to the design before it was manufactured for issue was that blade, guard, and pommel were all finished in a non-reflective matte black or grey phosphate finish instead of the brightly polished steel of the original. Marines today often give the blades, guards, and pommels of their knives a few coats of non-reflective matte black spray paint to reduce reflected light and give them a little more protection against saltwater corrosion.

The name came from the manufacturers of the knife, the Union Cutlery Company, who stamped their trademark into the mark side tang. More than one million of these knives were manufactured during WWII. The Ka-bar remains a favorite, virtually obligatory piece of equipment with US Marine infantry at present, and it has become a symbol of the Marines. The US Marine Reconnaissance units, however, generally prefer the Gerber Mark II . US Marine Corps basic training includes considerable time spent on knife fighting techniques, as well as other edged weapons.

The KA-BAR company was founded in 1898 as Tidioute Cutlery Company; following its financial collapse and take-over it was renamed the Union Cutlery Company. KA-BAR was initially a trademark but in 1952 the company renamed itself KA-BAR Cutlery Inc.

The origin of the word "Ka-bar" is uncertain. The company claims it is a corruption of "Kill a Bear", an ability ascribed to the knife by a customer. A competing etymology, displayed in a museum at the Marine Corps Officer Candidates School (OCS) in Quantico, VA, is that it originally stood for "Knife Attached - Browning Automatic Rifle."