Big Arm is a census-designated place (CDP) in Lake County, Montana, United States. The population was 131 at the 2000 census.
The town forms a big arm on Flathead Lake, hence its name. The name was used by the Kootenai who lived there prior to the town being established. The post office opened in 1911 with Marion F. Lamb as postmaster.
Big Arm State Park is located just north of the town.