On December 7, 1787, Delaware became the first state to ratify the Constitution. Article VII of the Constitution required that 9 out of the original 13 colonies ratify the document for it to be adopted. On June 21, the following year, New Hampshire became that 9th state. Rhode Island became the last state to ratify the Constitution nearly 2 years later in May of 1790.
Read more about the ratification process at the National Archives.