The Invention Secrecy Act of 1951 (Pub. L. 82–256, 66 Stat. 3, enacted February 1, 1952, codified at 35 U.S.C. ch. 17) is a body of United States federal law designed to prevent disclosure of new inventions and technologies that, in the opinion of selected federal agencies, present a possible threat to the national security of the United States.

The U.S. government has long sought to control the release of new technologies that might threaten the national defense and economic stability of the country. During World War I, Congress authorized the United States Patent and Trademark Office (PTO) to classify certain defense-related patents. This initial effort lasted only for the duration of that war but was reimposed in October 1941 in anticipation of the U.S. entry into World War II. Patent secrecy orders were initially intended to remain effective for two years, beginning on July 1, 1940, but were later extended for the duration of the war (Pub. L. 77–239: AN ACT To amend the Act relating to preventing the publication of inventions in the national interest, and for other purposes).

The Invention Secrecy Act of 1951 made such patent secrecy permanent, though the order to suppress any invention must be renewed each year (except during periods of declared war or national emergency). Under this Act, defense agencies provide the PTO with a classified list of sensitive technologies in the form of the "Patent Security Category Review List" (PSCRL). The decision to classify new inventions under this act is made by "defense agencies" as defined by the President. Generally, these agencies include the Army, Navy, Air Force, National Security Agency (NSA), Department of Energy, and NASA, but even the Justice Department has played this role.

A secrecy order bars the award of a patent, orders that the invention be kept secret, restricts the filing of foreign patents, and specifies procedures to prevent disclosure of ideas contained in the application. The only way an inventor can avoid the risk of such imposed secrecy is to forgo patent protection.

By the end of fiscal year 1991, the number of patent secrecy orders stood at 6,193. Many such orders were imposed on individuals and organizations working without government support. This number shrank for each fiscal year thereafter, until 2002. Since 2002, the number of secrecy orders has grown, with 5,002 secrecy orders in effect at the end of fiscal year 2007.