The Martini-Enfield Mark I is a historical British breech-loading rifle, originally adapted from the Martini-Henry rifle for the .303 British cartridge, a transition that marked the evolution of British military rifles at the end of the 19th century. This adaptation process began in the late 1880s and early 1890s.

The original Martini-Henry rifle, used extensively during the Anglo-Zulu War in 1879, was initially chambered in .577/450 Martini-Henry caliber. However, with the advent of smokeless powder and smaller-caliber bullets, which provided better range and accuracy, the British Army needed to update its weaponry to stay competitive.

The Martini-Enfield was essentially a Martini-Henry rifle that had been rebarreled or rechambered to fit the new .303 British cartridge, which was also used in the Lee-Metford and later Lee-Enfield rifles. The Mark I designation refers to the first model of this adaptation, featuring improvements such as a new sighting system and a longer barrel that could better handle the ballistic properties of the .303 cartridge.