The 9×21mm pistol cartridge (also known as the 9×21mm GP, 9×21mm IMI, 9mm IMI, 9×21mm Italian, or 9mm Italian) was designed by Jager (Loano, Italy), then adopted and commercialised by Israel Military Industries for those jurisdictions where military service cartridges, like the 9×19mm Parabellum, are or were illegal for civilian purchase (i.e. Italy, France, Brazil, and Mexico).

The bullet sits slightly deeper in the case, which results in almost the same overall length as the 9×19mm Parabellum cartridge (29.69 to 29.75 mm (1.169 to 1.171 in)).

A rimless case is preferred because, unlike semi-rimmed or rimmed cartridges, it will not tend to lock together with other rounds in a double-column magazine, resulting in a malfunction.

As other .38 Super–based rimless cartridges became available, the 9×21mm decreased in popularity in the United States for competition use, but it is still widely used in many other parts of the world.

(Commission Internationale Permanente pour l'épreuve des armes à feu portatives) guidelines, the 9×21mm case can handle up to 235 MPa (34,100 psi) piezo pressure.