The county was formed on December 14, 1798. It was derived from parts of Franklin and Shelby counties. Later, parts of the county were pared off to create three additional counties: Owen in 1819, Trimble in 1836 and Carroll in 1838. Today Gallatin is one tenth of its original size.
The county was named after Albert Gallatin a Swiss native, who became a financier, prominent American Statesman and
served as Secretary of the Treasury of President Thomas Jefferson.
Its county seat is Warsaw.