An acronym for the Defense Advanced Research Projects Agency of the United States. It is the R&D arm of the Pentagon (United States Department of Defense). It has been responsible for the development of various new technologies, and has made many contributions to military development. Its name was changed from ARPA in 1972. The organization is well-known for its development of ARPAnet, which became the basis of the modern-day Internet.

In 2005, DARPA director Donald Anderson established a military-industrial complex in conjunction with ArmsTech. In secret, they began development of a nuclear-capable bipedal tank (Metal Gear REX) on Shadow Moses Island. While conducting field tests on Metal Gear REX's stealth bomb capabilities, FOXHOUND, led by Liquid Snake, went renegade, instigating the Shadow Moses Incident.