The United States Declaration of Independence was an act of the Second Continental Congress, adopted on July 4, 1776, which declared that the Thirteen Colonies in North America were "Free and Independent States" and that "all political connection between them and the State of Great Britain, is and ought to be totally dissolved." The document, formally entitled The unanimous Declaration of the thirteen United States of America, explained the justifications for separation from the British crown, and was an expansion of Richard Henry Lee's Resolution (passed by Congress on July 2), which first proclaimed independence. An engrossed copy of the Declaration was signed by most of the delegates on August 2 and is now on display in the National Archives and Records Administration in Washington, D.C.

The Declaration is considered to be the founding document of the United States of America, where July 4 is celebrated as Independence Day and the nation's birthday. At the time the Declaration was issued, the American colonies were "united" in declaring their independence from Great Britain. John Hancock, as the elected President of Congress, was the only person to sign the Declaration of Independence on July 4th. It was not until the following month on August 2nd that the remaining 55 other delegates began to sign the document.

US President Abraham Lincoln succinctly explained the central importance of the Declaration to American history in his Gettysburg Address of 1863: