The state with the longest official name is also the smallest. The winner is the "State of Rhode Island and Providence Plantations." 42 letters, not counting spaces.
Despite the name, most of Rhode Island is on the mainland United States. The official name of the state, Rhode Island and Providence Plantations, derives from the merger of two settlements. Rhode Island colony was founded near present-day Newport, on what is now commonly called Aquidneck Island, the largest of several islands in Narragansett Bay. Providence Plantations was the name of the colony founded by Roger Williams in the area now known as the City of Providence.