Poplar is a city in Roosevelt County, Montana, United States. The population was 758 at the 2020 census. It is the tribal headquarters for the Fort Peck Indian Reservation, though Wolf Point is the most populous. The reservation is home to both the Assiniboine and Sioux Tribes, two distinct American Indian Nations. The U.S. Army constructed Camp Poplar here in the 1870s to oversee the reservation. Poplar became reservation headquarters after the military abandoned the camp in 1893.