The United States Constitution was signed on September 17, 1787, at Independence Hall in Philadelphia.
The signing concluded the Constitutional Convention, which had met in Philadelphia for four months to replace the Articles of Confederation with a stronger federal framework. Thirty-nine delegates endorsed the document. They represented twelve states because Rhode Island declined to send delegates to the convention. George Washington, who presided over the convention, signed along with figures including Benjamin Franklin, James Madison, and Alexander Hamilton.
Signing did not immediately put the Constitution into effect. The document required ratification by conventions in the states, and Article Seven specified that approval by nine states would be sufficient for it to take effect among the ratifying states. New Hampshire became the ninth state to ratify on June 21, 1788. The federal government under the Constitution began operating in 1789, making September 17 a major constitutional milestone within the broader America250 story.