What was the only state not represented at the Constitutional Convention in Philadelphia?

Rhode Island (The Providence Plantations of Rhode Island). The State's leaders refused to participate in the Constitutional Convention because they believed it was a conspiracy to overthrow the current government, which was operating under the Articles of Convention. Patrick Henry, who would have been their delegate, declared he "smelt a rat," and didn't want to participate in a project that threatened state sovereignty.

Rhode Island also became the last of the original states to ratify the Constitution, on May 29, 1790.