Thirty-nine delegates signed the United States Constitution on September 17, 1787. The signing occurred at Independence Hall in Philadelphia during the final session of the Constitutional Convention.
The convention had included 55 delegates at different points, representing 12 states; Rhode Island declined to participate. Some delegates left before the final signing, and three of those still present—George Mason, Edmund Randolph, and Elbridge Gerry—refused to sign. The 39 signers therefore represented only part of the convention's membership.
The Constitution still required ratification by conventions in the states. It took effect after New Hampshire became the ninth state to ratify it in June 1788, but the new federal government did not begin operating under it until March 4, 1789. The signing date is different from the ratification dates, a distinction often overlooked in accounts of the founding era.