Thurston County is a county located in the U.S. state of Washington. As of the 2020 census, its population was 294,793. The county seat is Olympia, the state capital.

Thurston County was created out of Lewis County by the government of Oregon Territory on January 12, 1852. At that time, it covered all of the Puget Sound region and the Olympic Peninsula. On December 22 of the same year, Pierce, King, Island, and Jefferson counties were split off from Thurston County. It is named after Samuel R. Thurston, the Oregon Territory's first delegate to Congress. Today, the county includes the southernmost part of the South Puget Sound and areas south along the I-5 corridor.

Thurston County comprises the Olympia–Lacey–Tumwater, WA Metropolitan Statistical Area and is included in the Seattle–Tacoma, WA Combined Statistical Area.

The southern end of Puget Sound is the homeland of several indigenous Coast Salish groups, including the Nisqually, Squaxin, and Upper Chehalis. Archeological remains at Tumwater Falls date back to 2,500 to 3,000 years before present; the area around the falls included a settlement with several longhouses. The first European exhibition to the southern Puget Sound was conducted by Peter Puget and Joseph Whidbey on the British-led Vancouver Expedition in May 1792. The Hudson's Bay Company established a trading post at Fort Nisqually in 1833 on the east side of the Nisqually Delta while the Oregon Country was under joint administration by the British and American governments. Permanent European (and later American) settlement of modern-day Thurston County began with the arrival of a pioneer party led by Michael Simmons and Black pioneer George Bush in 1845. Several families settled near Tumwater Falls at a site they named "New Market", which became the first European settlement in Western Washington.

The area north of the Columbia River was originally under the jurisdiction of the Vancouver District (later renamed Clark County) until 1845, when Lewis County was created from the area west of the Cowlitz River. The entire region was ceded to the United States with the signing of the Oregon Treaty in 1846 and organized into Oregon Territory two years later. A petition by 54 residents of Olympia and surrounding communities was submitted to the Oregon Territorial Legislature in December 1851 to create a new county from Lewis County. The proposed name of Simmons County, named for Michael Simmons, was changed to Thurston County by the legislature at the suggestion of Asa Lovejoy to honor Samuel Thurston, the first delegate to the U.S. Congress from Oregon Territory. Thurston himself had never visited the area.

Thurston County was created on January 12, 1852, by the Oregon Territorial Legislature and Olympia was designated as its seat. It included the entire Olympic Peninsula and Puget Sound region up to the northern border with British North America and went as far east as the Cascade Mountains. On December 22, the northern areas of Thurston County were divided to form Island, Jefferson, King, and Pierce counties. A portion of the county south of the Chehalis River was ceded to Lewis County in February 1853, a month before Washington Territory was created with its capital in Olympia. Sawamish County (now Mason County) was created in March 1854 from the northwestern portions of Thurston County and Chehalis County (now Grays Harbor County) was established a month later from the remaining western half of Thurston County. Several exchanges of land between Thurston and neighboring counties were made during the 1860s and settled into the modern boundaries by 1873. An attempt to move the county seat from Olympia to Tumwater or West Olympia was defeated by voters in 1861.

Olympia was retained as capital of Washington after it was granted statehood in 1889; the city did not win a majority in the first referendum after Ellensburg and North Yakima, but defeated both in a second vote. Local residents built a branch line to connect with the Northern Pacific Railroad and approved a harbor-dredging operation to promote Olympia as a trade hub as the area fell behind Seattle and Tacoma in population growth.

The 150-foot (46 m) Old Capitol Building was completed in 1892 and was purchased by the state government in 1901 for use to replace a temporary wooden structure built in 1856. The modern Washington State Capitol commenced construction in 1923 and was completed in 1928 alongside a campus of government buildings and monuments. Thurston County remained predominantly dependent on the logging industry until the state government became the county's largest employment sector in the 1950s. Several state government agencies had attempted to move their offices to Seattle until a 1954 Washington Supreme Court ruling mandated that their headquarters remain in the Olympia area.