The United States government classification system is established under Executive Order 13526, the latest in a long series of executive orders on the topic beginning in 1951. Issued by President Barack Obama in 2009, Executive Order 13526 replaced earlier executive orders on the topic and modified the regulations codified to 32 C.F.R. 2001. It lays out the system of classification, declassification, and handling of national security information generated by the U.S. government and its employees and contractors, as well as information received from other governments.

The desired degree of secrecy about such information is known as its sensitivity. Sensitivity is based upon a calculation of the damage to national security that the release of the information would cause. The United States has three levels of classification: Confidential, Secret, and Top Secret. Each level of classification indicates an increasing degree of sensitivity. Thus, if one holds a Top Secret security clearance, one is allowed to handle information up to the level of Top Secret, including Secret and Confidential information. If one holds a Secret clearance, one may not then handle Top Secret information, but may handle Secret and Confidential classified information.

The United States does not have a British-style Official Secrets Act. Instead, several laws protect classified information, including the Espionage Act of 1917, the Atomic Energy Act of 1954 and the Intelligence Identities Protection Act of 1982. A 2013 report to Congress noted that the relevant laws have been mostly used to prosecute foreign agents, or those passing classified information to them, and that leaks to the press have rarely been prosecuted. The legislative and executive branches of government, including US presidents, have frequently leaked classified information to journalists. Congress has repeatedly resisted or failed to pass a law that generally outlaws disclosing classified information. Most espionage law criminalizes only national defense information; only a jury can decide if a given document meets that criterion, and judges have repeatedly said that being "classified" does not necessarily make information become related to the "national defense". Furthermore, by law, information may not be classified merely because it would be embarrassing or to cover illegal activity; information may be classified only to protect national security objectives.

The United States over the past decades under the Obama and Clinton administrations has released classified information to foreign governments for diplomatic goodwill, known as declassification diplomacy. Examples include information on Augusto Pinochet to the government of Chile. In October 2015, US Secretary of State John Kerry provided Michelle Bachelet, Chile's president, with a pen drive containing hundreds of newly declassified documents.

A 2007 research report by Harvard history professor Peter Galison, published by the Federation of American Scientists, claimed that the classified universe in the US "is certainly not smaller and very probably is much larger than this unclassified one. ... [And] secrecy ... is a threat to democracy.