Thirty-nine delegates signed the United States Constitution on September 17, 1787.
The signatures came at the conclusion of the Constitutional Convention in Philadelphia. The convention had been called to address weaknesses in the Articles of Confederation, the first framework of government for the United States. Instead of merely revising that system, delegates produced an entirely new Constitution.
The convention included representatives from 12 states; Rhode Island declined to send delegates. Fifty-five delegates attended at some point, but not all supported the finished document. Three delegates—George Mason, Edmund Randolph, and Elbridge Gerry—refused to sign because they objected to aspects of the proposed government.
The number 39 therefore refers to signers, not attendees. The Constitution also needed state ratification. New Hampshire became the decisive ninth state to ratify it in June 1788, allowing the new federal government to begin operating in 1789.