when was the constitution adopted

The United States Constitution was adopted on September 17, 1787, in Philadelphia, when delegates at the Constitutional Convention signed the final document.

Key dates at a glance

• Drafting and signing (often called “adoption”) : September 17, 1787, when 39 delegates signed the finished Constitution at the Constitutional Convention in Philadelphia.

• Ratification as the official framework of government: June 21, 1788, when New Hampshire became the ninth state to ratify, making the Constitution legally effective under Article VII.

• All 13 original states completed ratification by May 29, 1790.

Why the date can be confusing

People sometimes mean different moments when they ask “when was the Constitution adopted”:

• Some focus on the signing date (September 17, 1787), when the text was agreed and endorsed by the convention.

• Others emphasize the ratification date (June 21, 1788), when it actually became the official basis of the U.S. government after nine states approved it.

If you’re asking for a school assignment, it usually expects the signing/adoption date of September 17, 1787 unless the question specifically says “when did it go into effect” or “when was it ratified.” Are you asking about the U.S. Constitution for a class assignment, a test prep context, or a general-history interest?