Big Arm is a census-designated place (CDP) in Lake County, Montana, United States.

The town is situated on the south side of Big Arm, a large bay on Flathead Lake, hence its name.

[5] Big Arm State Park is located just north of the town.

The census-designated place contains the settlement of Big Arm along the southern shore of Flathead Lake's Big Arm, as well as undeveloped grassland to the south, up to the base of an unnamed 5,100-foot (1,600 m) mountain.

U.S. Route 93 passes through Big Arm, running just south of the lake shore.

According to the United States Census Bureau, the CDP has a total area of 5.4 square miles (14.0 km2), all land.