St. Pierre is located in southeastern Hill County on the northwestern edge of the Bears Paw Mountains.

It is bordered to the south and west by Chouteau County, to the north by Sangrey, and to the east by Rocky Boy's Agency.

According to the United States Census Bureau, the St. Pierre CDP has a total area of 8.1 square miles (20.9 km2), all land.

9.2% of all households were made up of individuals, and none had someone living alone who was 65 years of age or older.

About 28.4% of families and 28.9% of the population were below the poverty line, including 36.8% of those under the age of eighteen and none of those 65 or over.