The USA PATRIOT Act, commonly known as the Patriot Act, is a controversial Act of Congress that U.S. President George W. Bush signed into law on October 26, 2001. The contrived acronym stands for "Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism Act of 2001" (Public Law USPL: 56).

The act expands the authority of U.S. law enforcement agencies for the stated purpose of fighting terrorism in the United States and abroad. Among its provisions, the Act increases the ability of law enforcement agencies to search telephone, e-mail communications, medical, financial and other records; eases restrictions on foreign intelligence gathering within the United States; expands the Secretary of the Treasury's authority to regulate financial transactions, particularly those involving foreign individuals and entities; and enhances the discretion of law enforcement and immigration authorities in detaining and deporting immigrants suspected of terrorism-related acts. The act also expands the definition of terrorism to include domestic terrorism, thus enlarging the number of activities to which the USA Patriot Act's expanded law enforcement powers can be applied.

The Act was passed by wide margins in both houses of Congress and was supported by members of both the Republican and Democratic parties. Despite widespread congressional support, it has been criticized for weakening protections of civil liberties. In particular, opponents of the law have criticized its authorization of indefinite detentions of immigrants; searches through which law enforcement officers search a home or business without the owner's or the occupant's permission or knowledge; the expanded use of National Security Letters, which allows the FBI to search telephone, email and financial records without a court order; and the expanded access of law enforcement agencies to business records, including library and financial records. Since its passage, several legal challenges have been brought against the act, and Federal courts have ruled that a number of provisions are unconstitutional.

Many of the act's provisions were to sunset beginning December 31, 2005, approximately 4 years after its passage. In the months preceding the sunset date, supporters of the act pushed to make its sunsetting provisions permanent, while critics sought to revise various sections to enhance civil liberty protections. In July 2005, the U.S. Senate passed a reauthorization bill with substantial changes to several sections of the act, while the House reauthorization bill kept most of the act's original language. The two bills were then reconciled in a conference committee that was criticized by Senators from both the Republican and Democratic parties for ignoring civil liberty concerns. The bill, which removed most of the changes from the Senate version, passed Congress on March 2, 2006 and was signed into law by President George W. Bush on March 9, 2006.