The US Constitution was adopted on the 17th of September, 1787, by the Constitutional Convention in Philadelphia, Pennsylvania. Article Seven of the Constitution required that at least nine states must ratify the constitution in order for it to come into effect. The first state to ratify the Constitution was Delaware on 7th December, 1787. The ninth was New Hampshire on the 21st of June, 1788. The last of the 13 states to ratify was Rhode Island on the 29th of May 1790.