The .300 Remington Ultra Magnum, also known as the .300 Ultra Mag-300RUM is a 7.62 mm (.308in.) caliber rifle cartridge, 7.62×72mm, or .30 caliber rifle cartridge introduced by Remington Arms in 1999. The .300 Remington Ultra Magnum is one of the largest commercially available .30 caliber magnums currently being produced. It is a beltless, rebated rim cartridge, capable of handling all large North American game, as well as long-range shooting. Among commercially produced .30-caliber rifle chamberings, the .300 Remington Ultra Magnum is second only to the .30-378 Weatherby Magnum in cartridge-case capacity.

The .300 Ultra Mag-300RUM cartridge was conceived as a long range hunting cartridge and in this it excels most other cartridges. It is able to launch bullets of good weights and sectional densities at high velocities and remaining energy to take game cleanly at longer ranges than lesser cartridges such as the .30-06 Springfield and even the .300 Winchester Magnum.

The .300 RUM is an excellent long range cartridge with the ability to deliver a useful level of energy downrange especially with the power level III ammunition. Due to its high velocity it exhibits less bullet drop than most other .30 caliber (7.62 mm) cartridges.