Keith Brian Alexander (born 2 December 1951) is a retired four-star general of the United States Army, who served as director of the National Security Agency, chief of the Central Security Service, and commander of the United States Cyber Command (CYBERCOM). He previously served as Deputy Chief of Staff, G-2 (Intelligence), United States Army from 2003 to 2005. He assumed the positions of Director of the National Security Agency and Chief of the Central Security Service on 1 August 2005, and the additional duties as Commander United States Cyber Command on 21 May 2010.

Alexander announced his retirement on 16 October 2013. His retirement date was 28 March 2014. In May 2014, Alexander founded IronNet Cybersecurity, a private-sector cybersecurity firm based in Fulton, Maryland. He would leave the company in February 2024.

Alexander was born on 2 December 1951, in Syracuse, New York, the son of Charlotte L. (Colvin) and Donald Henry Alexander. He was raised in Onondaga Hill, New York, a suburb of Syracuse. He was a paperboy for The Post-Standard and attended Westhill Senior High School, where he ran track.

Alexander attended the United States Military Academy at West Point, and in his class were three other future four-star generals: David Petraeus, Martin Dempsey and Walter L. Sharp. In April 1974,^{[dubious – discuss]} Alexander married Deborah Lynn Douglas, who was a classmate in high school and who grew up near his family in Onondaga Hill. They had four daughters.

Alexander entered active duty at West Point, intending to serve for only five years. Alexander's military education includes the Armor Officer Basic Course, the Military Intelligence Officer Advanced Course, the United States Army Command and General Staff College, and the National War College.

Alexander worked on signals intelligence at a number of secret National Security Agency bases in the United States and Germany. He earned a Master of Science in business administration in 1978 from Boston University, a Master of Science in systems technology (electronic warfare) and a Master in Science in physics in 1983 from the Naval Postgraduate School, and a Master of Science in national security strategy from the National Defense University. He rose quickly up the military ranks, due to his expertise in advanced technology and his competency at administration.

Alexander's assignments include the Deputy Chief of Staff (DCS, G-2), Headquarters, Department of the Army, Washington, D.C., from 2003 to 2005; Commanding General of the United States Army Intelligence and Security Command at Fort Belvoir, Virginia, from 2001 to 2003; Director of Intelligence (J-2), United States Central Command, MacDill Air Force Base, Florida, from 1998 to 2001; and Deputy Director for Intelligence (J-2) for the Joint Chiefs of Staff from 1997 to 1998. Alexander served in a variety of command assignments in Germany and the United States. These include tours as Commander of Border Field Office, 511th MI Battalion, 66th MI Group; 336th Army Security Agency Company, 525th MI Group; 204th MI Battalion; and 525th Military Intelligence Brigade.

Additionally, Alexander held key staff assignments as Deputy Director and Operations Officer, Executive Officer, 522nd MI Battalion, 2nd Armored Division; G-2 for the 1st Armored Division both in Germany and during the Gulf War, in Operation Desert Shield and Operation Desert Storm, in Saudi Arabia. He also served in Afghanistan on a peace keeping mission for the Army Deputy Chief of Staff for Intelligence.