Gallatin County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 89,513, making it the fourth-most populous county in Montana. Its county seat is Bozeman, home of Montana State University. The county was founded in 1865.Gallatin County comprises the Bozeman, MT Micropolitan Statistical Area.The prominent physical feature of the county is the Gallatin River, named by Meriwether Lewis in 1805 for Albert Gallatin, U.S. Treasury Secretary from 1801-14.