The .30-378 Weatherby Magnum is a .30 caliber, belted, bottle-necked rifle cartridge. The cartridge was developed in response to a US Army military contract in 1959. While still unreleased to the public, the cartridge went on to set world records for accuracy including the first ten 10X in 1,000 yards (910 m) benchrest shooting. It is currently the highest velocity .30 caliber factory ammunition available.

The .378 Weatherby Magnum was designed by Roy Weatherby in 1953. It was an original belted magnum design with no parent case, inspired by the .416 Rigby and headspacing of the belted .375 H&H Magnum. The 215 magnum rifle primer was developed by Federal specifically for this round. The cartridge can hold upwards of 7.13 g of powder. The 378 Weatherby Magnum cartridge also has the double radius shoulder design found on the first and smaller proprietary line of Weatherby magnum cartridges.