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.

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.

The city of Kalispell is 39 miles (63 km) north of Big Arm.

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