The FN 5.7×28mm (designated as the 5.7×28 by the C.I.P. and FN 5.7x28mm NATO) is a small-caliber, high-velocity, smokeless-powder, rebated-rim, bottlenecked centerfire cartridge designed for handgun and personal defense weapon (PDW) uses, manufactured by FN Herstal. It is similar in length to the .22 WMR (5.7×27mm) and to some degree similar also to the .22 Hornet or .22 K-Hornet. Unlike many new cartridges, it has no parent case; the complete package was developed from scratch by FN.
The 5.7×28mm was developed in conjunction with the FN P90 PDW and FN Five-seven pistol in response to NATO requests for a replacement for the 9×19mm Parabellum cartridge. In 2002 and 2003, NATO conducted a series of tests with the intention of standardizing a PDW cartridge as a replacement for the 9×19mm Parabellum cartridge. The tests compared the relative merits of the 5.7×28mm cartridge and the 4.6×30mm cartridge, which was created by Heckler & Koch as a competitor to the 5.7×28mm. The NATO group subsequently recommended the 5.7×28mm cartridge, citing superior performance in testing, but the German delegation objected and the standardization process was halted until 2021 when it was officially adopted as a NATO standard STANAG 4509.