Delegates signed the United States Constitution on September 17, 1787.
The signing took place at the Pennsylvania State House, now Independence Hall, in Philadelphia. Thirty-nine of the 42 delegates still present signed the document. George Washington, who presided over the Constitutional Convention, signed first, followed by the other delegates.
The convention had met from May through September 1787. Its original assignment was to revise the Articles of Confederation, but the delegates instead designed an entirely new framework for the federal government. The Constitution created separate legislative, executive, and judicial branches and divided powers between the national government and the states.
Signing did not immediately make the Constitution law. Ratification by nine states was required, and that threshold was reached when New Hampshire ratified on June 21, 1788. The new federal government began operating in 1789.