Tumwater is a city in Thurston County, Washington, United States. The population was 25,350 at the 2020 census. The city is situated near where the Deschutes River enters Budd Inlet, the southernmost point of Puget Sound; it also borders the state capital of Olympia to the north. Tumwater is the oldest permanent Anglo-American settlement on Puget Sound. The city is the southern gateway to the wider Seattle/Tacoma urban area, which stretches from Tumwater to Marysville along the Interstate 5 corridor.