Jefferson County Courthouse was built on Arsenal Street in Watertown in 1862. Designed by architect Horatio Nelson White, it is one of three county courthouses designed by White in the similar style. The structure remained the county courthouse until 2004, when the county moved the courthouse to the renovated Watertown Post Office Building. After three years of restoration work, the courthouse is now home to the Jefferson County Legislature and several other county departments.
The courthouse was added to the National Register of Historic Places in 1974.