The Black River is a river in Thurston County in the U.S. state of Washington. It is approximately 25 miles long with a drainage basin of about 144 square miles. In general, the Black River is a slow flowing river with a broad floodplain. Land along the Black River contains one of the largest remaining wetland systems in western Washington, with extensive areas of unique, undisturbed swamp, containing prime habitat for migratory birds and other wildlife. The Black River supports robust runs of chum, Chinook and coho salmon, in addition to steelhead and cutthroat trout. 

The fur trader John Work was the first to describe the Black River in 1824: "The Black River so named from the color of its water … A great many dead salmon are in the river, and many that are just alive and barely able to move through the water." 

The Black River's source is Black Lake, located about 3 miles (4.8 km) west of Tumwater. The river flows generally south, through Littlerock, near the Mima Mounds Natural Area Preserve, then southwest, passing through the Black River Habitat Management Area and the town of Rochester, before meandering west through the community of Gate and entering Grays Harbor County, where it empties into the Chehalis River. The Black River drainage is divided into two uniquely different halves. The west side drains the Capitol Forest area and the main tributaries are Dempsey, Waddell, and Mima creeks. This area ranges in elevation from 2,659 feet at Capitol Peak down to approximately 200 feet at the Black River valley floor. The east side of the watershed drains the relatively flat area south of Tumwater, west of Offutt Lake and north of Tenino. The elevation difference of this area is approximately 200 feet. The main streams draining this side are Salmon and Beaver creeks and Bloom Ditch.