The .400 Jeffery Nitro Express or .450/400 Nitro Express 3-inch is a medium bore, bottlenecked, Nitro Express cartridge designed by W.J. Jeffery & Co in 1902, intended for use in single shot and double rifles.

The Nitro Express (NE) series of cartridges are used in large-bore hunting rifles, also known as elephant guns or express rifles, but later came to include smaller bore high velocity British cartridges.