The Black River is a river in Thurston County in the U.S. state of Washington. It is approximately 25 miles (40 km) long with a drainage basin of about 144 square miles (370 km2). The Black River's source is Black Lake, located about 3 miles (4.8 km) west of Tumwater. The river flows generally south, through the Mima Mounds State Natural Area, then southwest, passing by Rochester and entering Grays Harbor County, then emptying into the Chehalis River in the Chehalis Indian Reservation.