The U.S. Constitution was signed at the Philadelphia Convention in 1787.
The signing took place on September 17, 1787, at Independence Hall in Philadelphia, Pennsylvania. Thirty-nine delegates endorsed the document at the convention’s final session, although several delegates had already departed and three attending delegates refused to sign.
The Constitutional Convention had met for about four months. It was originally called to revise the Articles of Confederation, but the delegates instead created an entirely new framework for the federal government. The Constitution established three branches of government and a system of federalism.
The signing did not immediately make the Constitution effective. Under Article VII, nine states had to ratify it. New Hampshire became the ninth state to ratify on June 21, 1788, and the new federal government began operating on March 4, 1789. A common mix-up is confusing the signing date with the ratification date or the date the government began.