The Poplar River is a tributary of the Missouri River, approximately 167 miles (269 km) long in Saskatchewan in Canada and Montana in the United States. It rises in several forks on the plains of southern Saskatchewan. The western fork rises near Killdeer, Saskatchewan and flows southeast, into northeastern Montana, past Richland and across Daniels County. The middle fork rises northwest of Rockglen, Saskatchewan and flows southeast, into northeastern Montana, and passes west of Scobey. The two forks unite in the northern Fort Peck Indian Reservation. The combined fork flows southeast, then south-southwest, and joins the Missouri River near Poplar.