The United Defense M42, sometimes known as the Marlin for the company that did the actual manufacturing, was an American submachine gun used during World War II. It was produced from 1942 to 1943 by United Defense Supply Corp. for possible issue as a replacement for the Thompson submachine gun and was used by Office of Strategic Services (OSS) agents. However, its usage was limited, and the Thompson continued to see service until the end of the war, alongside the M3 submachine gun, which was designed around the same time as the M42.