The National Security Agency (NSA) is an intelligence agency of the United States Department of Defense, under the authority of the director of national intelligence (DNI). The NSA is responsible for global monitoring, collection, and processing of information and data for global intelligence and counterintelligence purposes, specializing in a discipline known as signals intelligence (SIGINT). The NSA is also tasked with the protection of U.S. communications networks and information systems. The NSA relies on a variety of measures to accomplish its mission, the majority of which are clandestine. The NSA has roughly 32,000 employees.

Originating as a unit to decipher coded communications in World War II, it was officially formed as the NSA by President Harry S. Truman in 1952. Between then and the end of the Cold War, it became the largest of the U.S. intelligence organizations in terms of personnel and budget. Still, information available as of 2013 indicates that the Central Intelligence Agency (CIA) pulled ahead in this regard, with a budget of $14.7 billion. The NSA currently conducts worldwide mass data collection and has been known to physically bug electronic systems as one method to this end. The NSA is also alleged to have been behind such attack software as Stuxnet, which severely damaged Iran's nuclear program. The NSA, alongside the CIA, maintains a physical presence in many countries across the globe; the CIA/NSA joint Special Collection Service (a highly classified intelligence team) inserts eavesdropping devices in high-value targets (such as presidential palaces or embassies). SCS collection tactics allegedly encompass "close surveillance, burglary, wiretapping, [and] breaking".

Unlike the CIA and the Defense Intelligence Agency (DIA), both of which specialize primarily in foreign human espionage, the NSA does not publicly conduct human intelligence gathering. The NSA is entrusted with assisting with and coordinating SIGINT elements for other government organizations—which Executive Order prevents from engaging in such activities on their own. As part of these responsibilities, the agency has a co-located organization called the Central Security Service (CSS), which facilitates cooperation between the NSA and other U.S. defense cryptanalysis components. To further ensure streamlined communication between the signals intelligence community divisions, the NSA director simultaneously serves as the Commander of the United States Cyber Command and as Chief of the Central Security Service.

The NSA's actions have been a matter of political controversy on several occasions, including its role in providing intelligence during the Gulf of Tonkin incident, which contributed to the escalation of U.S. involvement in the Vietnam War. Declassified documents later revealed that the NSA misinterpreted or overstated signals intelligence, leading to reports of a second North Vietnamese attack that likely never occurred. The agency has also received scrutiny for spying on anti–Vietnam War leaders and the agency's participation in economic espionage. In 2013, the NSA had many of its secret surveillance programs revealed to the public by Edward Snowden, a former NSA contractor. According to the leaked documents, the NSA intercepts and stores the communications of over a billion people worldwide, including United States citizens. The documents also revealed that the NSA tracks hundreds of millions of people's movements using cell phone metadata. Internationally, research has pointed to the NSA's ability to surveil the domestic Internet traffic of foreign countries through "boomerang routing".

The origins of the National Security Agency can be traced back to April 28, 1917, three weeks after the U.S. Congress declared war on Germany in World War I. A code and cipher decryption unit was established as the Cable and Telegraph Section, which was also known as the Cipher Bureau. It was headquartered in Washington, D.C., and was part of the war effort under the executive branch without direct congressional authorization. During the war, it was relocated in the army's organizational chart several times. On July 5, 1917, Herbert O. Yardley was assigned to head the unit. At that point, the unit consisted of Yardley and two civilian clerks. It absorbed the Navy's cryptanalysis functions in July 1918. World War I ended on November 11, 1918, and the army cryptographic section of Military Intelligence (MI-8) moved to New York City on May 20, 1919, where it continued intelligence activities as the Code Compilation Company under the direction of Yardley.

After the disbandment of the U.S. Army cryptographic section of military intelligence known as MI-8, the U.S. government created the Cipher Bureau, also known as Black Chamber, in 1919. The Black Chamber was the United States' first peacetime cryptanalytic organization. Jointly funded by the Army and the State Department, the Cipher Bureau was disguised as a New York City commercial code company; it produced and sold such codes for business use. Its true mission, however, was to break the communications (chiefly diplomatic) of other nations. At the Washington Naval Conference, it aided American negotiators by providing them with the decrypted traffic of many of the conference delegations, including the Japanese. The Black Chamber successfully persuaded Western Union, the largest U.S. telegram company at the time, as well as several other communications companies, to illegally give the Black Chamber access to cable traffic of foreign embassies and consulates. Soon, these companies publicly discontinued their collaboration. Despite the Chamber's initial successes, it was shut down in 1929 by U.S. Secretary of State Henry L. Stimson, who defended his decision by stating, "Gentlemen do not read each other's mail."

During World War II, the Signal Intelligence Service (SIS) was created to intercept and decipher the communications of the Axis powers. When the war ended, the SIS was reorganized as the Army Security Agency (ASA), and it was placed under the leadership of the Director of Military Intelligence.

On May 20, 1949, all cryptologic activities were centralized under a national organization called the Armed Forces Security Agency (AFSA). This organization was originally established within the U.S. Department of Defense under the command of the Joint Chiefs of Staff. The AFSA was tasked with directing the Department of Defense communications and electronic intelligence activities, except those of U.S. military intelligence units. However, the AFSA was unable to centralize communications intelligence and failed to coordinate with civilian agencies that shared its interests, such as the Department of State, the Central Intelligence Agency (CIA) and the Federal Bureau of Investigation (FBI). In December 1951, President Harry S. Truman ordered a panel to investigate how AFSA had failed to achieve its goals. The results of the investigation led to improvements and its redesignation as the National Security Agency.