The .357 Smith & Wesson Magnum, .357 S&W Magnum, .357 Magnum, or 9×33mmR is a smokeless powder cartridge with a 0.357 in (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.

Elmer Merrifield Keith was an American rancher, firearms enthusiast, and author. Keith was instrumental in the development of the first magnum revolver cartridge, the .357 Magnum (1935), as well as the later .44 Magnum (1956) and .41 Magnum (1964) cartridges, credited by Roy G. Jinks as "the father of big bore handgunning." Keith was born in Hardin, Missouri, and overcame serious injuries that he had sustained at age 12 in a fire when he was living in Missoula, Montana.