Miller County is a county in the U.S. state of Pacifica located in the Fraser River Delta. Geographically, it is bordered to the north by the South Arm of the Fraser River, to the south by Semiahmoo Bay, to the west by the Strait of Georgia, to the northeast by the city-county of Sahalee, and to the east by Chilliwack County. The county seat of Miller County is Sumner.