The .338 Remington Ultra Magnum is a .338 caliber rifle cartridge introduced by Remington Arms in 2002.

It is a beltless, rebated rim cartridge based on the .300 Remington Ultra Magnum^[1] case shortened .090" and necked-up to accept a 0.338-inch (.338 caliber) bullet. The .338 Remington Ultra Magnum has a similar case capacity as the .338 Lapua Magnum^[2] and somewhat lower than that of the .338-378 Weatherby Magnum. It is one of the most powerful .338-caliber rounds in production.^{[citation needed]}

Because this cartridge already operates at very high pressures (65,000 PSI), handloaders cannot realize significant velocity improvements over factory ammunition as many handloaders have done over the years with more conventional, lower pressure rounds. However, they can still tune their own loads for best precision in their specific rifles, as with any other cartridge.^{[citation needed]}

• List of rifle cartridges
• Table of handgun and rifle cartridges
• .338 Edge