Spokane County is a county located in the U.S. state of Washington. As of the 2020 census, its population was 539,339, making it the fourth-most populous county in Washington. The largest city and county seat is Spokane, the second largest city in the state after Seattle. The county is named after the Spokane people.

Spokane County is part of the Spokane metropolitan area, which is also part of the greater Spokane–Coeur d'Alene combined statistical area that includes nearby Kootenai County, Idaho.

The first humans to arrive in what is now Spokane County arrived between 12,000 and 8,000 years ago and were hunter-gatherer societies who lived off the plentiful game in the area. Initially, the settlers hunted predominantly bison and antelope, but after the game migrated out of the region, the native people became dependent on gathering various roots, berries, and nuts, and harvesting fish. The Spokane tribe, after which the county is named, means "Children of the Sun" or "sun people" in Salishan Explorer-geographer David Thompson, working as head of the North West Company's Columbia Department, became the first European to explore what is now the Inland Northwest. After establishing the Kullyspell House and Saleesh House fur trading posts in what are now Idaho and Montana, Thompson then attempted to expand further west. He sent out two trappers, Jacques Raphael Finlay and Finan McDonald, to construct a fur-trading post on the Spokane River in Washington and trade with the local Indians. This post was established in 1810, at the confluence of the Little Spokane and Spokane Rivers, becoming the first enduring European settlement of significance in Washington. Known as the Spokane House, or simply "Spokane", it was in operation from 1810 to 1826.

Spokane County was established by the Washington Territorial Legislature effective January 29, 1858, from a portion of Walla Walla County, which originally encompassed most of eastern Washington Territory between the Cascades and Rockies. The new county was bound to the west by the Columbia and Snake rivers and to the east by the Rockies; it included portions of modern-day Idaho and Montana. The territorial legislature designated the farm of Angus McLeod as the temporary county seat and appointed officials to several positions for Spokane County, but they never took office and did not organize a government. In late 1859, a group of settlers in the Bitterroot Valley petitioned to create their own county, which was not granted at that time; the territorial legislature reorganized Spokane County on January 17, 1860, with a seat on a land claim near Fort Colville.

The first county government met on May 8, 1860, and began conducting business. The eastern and southern portions of Spokane County were partitioned several times as new counties were created, beginning with Missoula County in December 1860 and followed by Shoshone County and Nez Perce County in 1861. These areas became part of the new Idaho Territory, which was organized by the U.S. Congress on March 3, 1863, and reduced the size of Spokane County even further. On January 19, 1864, the county was annexed into neighboring Stevens County, which had been created a year earlier from the northern portions of Walla Walla County. The seat of Stevens County was Pinkney City (now Colville) until it was temporarily relocated to the town of Spokane Falls (now Spokane) in 1875.

Spokane County was re-established on October 30, 1879, from the portions of Stevens County south of the Columbia, Spokane, and Wenatchee rivers. The western portion of the county was used to create Lincoln County, which was established on November 23, 1883. The first post office in the county was located at Spokane Bridge.

The selection of a permanent county seat was to be decided in an election in November 1880 between the growing cities of Cheney and Spokane Falls, both candidates for a major Northern Pacific Railway hub. The unofficial returns showed a 14-vote margin in favor of Cheney, but the result was disputed by county officials from Spokane Falls based on "irregularities" in the ballots. The official result had a margin of two or three votes for Spokane Falls, but Cheney residents demanded a recount, which was granted by a court order that was ignored by county officials in Spokane Falls. On March 21, 1881, a group of armed Cheney residents forcibly took custody of the county auditor, recount ballots, and other county records during a nighttime raid. After declaring their own recount had been in favor of Cheney as county seat, the records and the county auditor were moved from Spokane Falls; other government officials also moved to Cheney after a court order upheld the Cheney recount. A new ballot question in 1886 resulted in Spokane becoming the permanent county seat.

According to the United States Census Bureau, the county has a total area of 1,781 sq mi (4,610 km²), of which 17 square miles (44 km²) (0.9%) are covered by water. The lowest point in the county is the Spokane River behind Long Lake Dam (boundary of Stevens County) at 1,538 feet (469 m) above sea level. (Virtually no change in elevation occurs between the dam and the mouth of the Little Spokane River inside Riverside State Park.) The highest point in the county is the summit of Mount Spokane at 5,887 ft (1,794 m). NAVD 88