The 9x18mm Ultra is a German pistol cartridge.  It was originally developed in the 1936 for use by the Luftwaffe, but was not adopted at that time.  In the 1972–1973 time frame, Walther introduced the Walther PP Super chambered in the cartridge for the West German Police. It might have been influenced by a relative success of Soviet 9×18mm Makarov, being an intermediate round between 9×17mm and 9×19mm Parabellum, fit for simple blowback pistols. The Walther PP Super was discontinued in 1979.  The cartridge was made available to the civilian market in 1975.   It did not gain lasting popularity however. 

In addition to the Walther PP Super, the SIG Sauer P230, Mauser HSc-80, and Benelli B76 were also produced in 9×18mm Ultra.

The 9×18mm Ultra (.355-inch caliber) is not interchangeable with the 9×18mm Makarov (.365-inch caliber), as the latter uses a larger diameter bullet.