Brockton is a census-designated place and former town in Roosevelt County, Montana, United States. The population was 358 at the 2020 census. Brockton's first post office was established in 1904. Fort Peck Indian Reservation was opened to non-Native settlers in 1913 and the town boomed. It was incorporated in 1952, and was disincorporated on January 23, 2018.