The Chehalis River (/ʃəˈheɪlɪs/ ^ⓘ shə-HAY-liss) is a river in Washington in the United States. It originates in several forks in southwestern Washington, flows east, then north, then west, in a large curve, before emptying into Grays Harbor, an estuary of the Pacific Ocean. The river is the largest solely contained drainage basin in the state.

The river was once much larger during the Ice Age when the tongue of the glacial ice sheet covering the Puget Sound terminated near Olympia and glacial runoff formed a large torrent of meltwater. This carved a large oversized valley that is much larger than the current river could have produced. The river's mouth was out near current Westport until rising sea levels at the end of the ice age flooded the broad Chehalis Valley to form a ria, known today as Grays Harbor.

The glacial sheet tongue is known as the Puget Lobe which, when it began to melt, formed Glacial Lake Russell. The lake drained through the Chehalis River Valley and the slow deposits of glacial sediment raised the depressed valley.

The Quinault Indian Nation and the Confederated Tribes of the Chehalis Reservation are stakeholders of the river. Though the people ceded the lands surrounding the Chehalis River upon the signing of the 1856 Treaty of Olympia, the tribes have retained fishing and hatchery rights.

Plans were raised during the presidency of Franklin Pierce to use the river as part of a canal stretching from Olympia to Grays Harbor. The idea was reintroduced multiple times during the 19th century but no official acts, nor construction of the waterway, materialized. Versions of the canal project persisted after the build of the Panama Canal and during the Great Depression, with scaled-down plans lasting into the 1970s.

During the Great Coastal Gale of 2007, a 20-mile (32 km) stretch of Interstate 5 was closed between exits 68 and 88 because of flooding from the Chehalis River, causing the roadway to be under about 10 feet (3.0 m) of water. The recommended detour added about four hours and 280 miles (450 km). It was not expected to reopen for several days. However, upon breaching a dike on Dec. 5, 2007, the water receded more quickly than anticipated. Amtrak train service between Portland, Oregon, and Vancouver, British Columbia, was also disrupted. Washington governor Christine Gregoire declared a state of emergency on December 3.

During the January 7, 2009, Pacific Northwest storms, a 20-mile (32 km) stretch of Interstate 5 was closed in and around the cities of Centralia and Chehalis because of flooding from the Chehalis River, causing the roadway to be under several feet of water. Since the main east–west mountain passes were also closed during this event, the flooding from the Chehalis River essentially cut off interstate traffic to the Puget Sound area from the south, and no detour was available.

The Chehalis River, along with the Dungeness River, is part of only two river basins in Washington state that are granted protections and rights under "in-stream flow regulation". Passed in 1976, the law allows the river the right to maintain its own water levels. Unfettered access to the river is granted to grandfathered "senior" rights holders as they existed before the 1976 rule went into effect; the senior holders mostly consist of tribal communities and farmers. As of 2023^[update], there are 93 recorded junior water rights holders, mostly homeowners, in the Chehalis basin.