[1] 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,[2] the United States Treasury Secretary who formulated the Lewis and Clark Expedition.

Big Sky Resort, one of the largest ski resorts in the United States, lies in Gallatin and neighboring Madison counties, midway between Bozeman and West Yellowstone.

There were 52,835 housing units in the county at an average density of 20.1 per square mile (7.8/km2).

[11] In terms of ancestry, 25.4% were German, 14.8% were Irish, 12.4% were English, 8.8% were Norwegian, 4.6% were Italian, 4.0% were French, 3.5% were Scottish, 2.2% were Polish and 0.3% were Subsaharan African.

In terms of ancestry, 32.2% were German, 18.1% were Irish, 14.7% were English, 9.4% were Norwegian, and 3.5% were American.

Gallatin County has traditionally favored Republican presidential candidates, often by large margins.

Recent changes to Gallatin County's demographics (including migration from Democratic-leaning areas,[12] and younger voters adopting more liberal political positions)[13] have made the county more competitive.

Gallatin County is home to the flagship campus of Montana State University, which contributes to its Democratic lean.

Democratic candidates Barack Obama and Hillary Clinton narrowly won Gallatin County in 2008 and 2016 respectively, with Republican Mitt Romney winning in 2012.