The United States Army Security Agency (ASA) was the United States Army's signals intelligence branch from 1945 to 1976. The Latin motto of the Army Security Agency was Semper Vigiles (Vigilant Always), which echoes the declaration, often mistakenly attributed to Thomas Jefferson, that "The price of liberty is eternal vigilance."

Although most ASA units focused upon SIGINT (signals intelligence) most if not all ASA units contained HUMINT (human intelligence) specialists as well, mostly interrogators and counter-intelligence specialists. At the end of the Cold War era, some ASA units also were staffed with ELINT (electronic intelligence) specialists and warrant officers, which incorporated field ECM (electronic counter-measures and field ECCM (electronic counter-countermeasures) such as tactical jammers, direction finders, electronic signal decoys, and captured/repurposed Warsaw Pact radio and communications equipment.

The Agency existed between 1945 and 1977 and was the successor to the Army Signals Intelligence Service, operations that dated to World War I. ASA was under the operational control of the Director of the National Security Agency (DIRNSA), located at Fort Meade, Maryland. It had its own tactical commander at Headquarters, ASA, at Arlington Hall Station, Virginia. Besides intelligence gathering, it had responsibility for the security of Army communications and for electronic countermeasures operations. In 1977, the ASA was merged with the US Army's Military Intelligence component to create the United States Army Intelligence and Security Command (INSCOM). However, the last separate ASA field unit (the 523rd ASA, an Army Reserve unit based at Fort Snelling, Minnesota) existed until 1976.