The .357 Smith & Wesson Magnum, .357 S&W Magnum, .357 Magnum, or 9×33mmR as it is known in unofficial metric designation, is a smokeless powder cartridge with a .357-inch (9.07 mm) bullet diameter. It was created by Elmer Keith, Phillip B. Sharpe, and Douglas B. Wesson of firearm manufacturers Smith & Wesson and Winchester. The .357 Magnum cartridge is notable for its highly effective terminal ballistics.
The .357 Magnum cartridge is based upon Smith & Wesson's earlier .38 Special cartridge. It was introduced in 1935, and its use has since become widespread. This cartridge began the "Magnum era" of handgun ammunition. The "Magnum era" began with the .375 H&H rifle cartridge, spreading to handguns with the .357 Magnum.