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. A small part of Yellowstone National Park lies in the southeastern portion of the county. The Big Sky Resort is about midway between West Yellowstone and Bozeman, though the ski area itself lies mostly in neighboring Madison County, Montana. The Gallatin River canyon is accessed by U.S. Highway 191. In the mid-19th century, a portion of Gallatin County was left over after the division of the Dakota Territory into other states. In 1873, the patch was attached to Montana and became part of Gallatin County.