Maine is the only US state that borders only one other state--New
Hampshire.

All contiguous US states touch at least two other states, with the
exception of one: Maine. Maine is the only state in the contiguous US that
only shares its border with one other state. Located in the most
northeastern part of the US, only Maine’s west side touches New
Hampshire. Other than its western border with New Hampshire, Maine has a
border with Canada on the north, and its eastern border is the Atlantic
Ocean. This proximity to the ocean plays a part in the state being the
leading lobster supplier in the US, accounting for over 90% of the
nation’s lobster.