A security agency is a governmental organization which conducts intelligence activities for the internal security of a nation. They are the domestic cousins of foreign intelligence agencies. For example, the United States Federal Bureau of Investigation (FBI) is a security agency, while the Central Intelligence Agency (CIA) is a foreign intelligence agency, which deals with the country's external security and intelligence. Security agencies frequently have "security", "intelligence" or "service" in their names. Private organizations that provide services similar to a security agency might be called a "security company" or "security service", but those terms can also be used for organizations that have nothing to do with intelligence gathering.

Säkerhetstjänst kallas det organ i ett land som har till uppgift att insamla uppgifter om människor i det egna landet i syfte att skydda mot brott mot rikets inre säkerhet.