Gallatin County is located in the U.S. state of Montana. With its county seat in Bozeman, it is the second-most populous county in Montana, with a population of 118,960 in the 2020 Census. The county's prominent geographical features are the Bridger mountains in the north, and the Gallatin mountains and Gallatin River in the south, named by Meriwether Lewis in 1805 for Albert Gallatin, the United States Treasury Secretary who formulated the Lewis and Clark Expedition.