Map - Multnomah County, Oregon (Multnomah County)

Multnomah County is one of the 36 counties in the U.S. state of Oregon. As of the 2020 census, the county's population was 815,428. Multnomah County is part of the Portland–Vancouver–Hillsboro, OR–WA Metropolitan Statistical Area. Though smallest in area, Multnomah County is the state's most populous county. Its county seat, Portland, is the state's largest city.

The area of the lower Willamette River has been inhabited for thousands of years, including by the Multnomah band of Chinookan peoples long before European contact, as evidenced by the nearby Cathlapotle village, just downstream.

Multnomah County (the thirteenth in Oregon Territory) was created on December 22, 1854, formed out of two other Oregon counties – the eastern part of Washington County and the northern part of Clackamas County. Its creation was a result of a petition earlier that year by businessmen in Portland complaining of the inconvenient location of the Washington County seat in Hillsboro and of the share of Portland tax revenues leaving the city to support Washington County farmers. County commissioners met for the first time on January 17, 1855.

The county is named after the Chinookan word for the "lower river", Multnomah, Matlnomaq, máɬnumax̣ being interpretive English spellings of the same word. In Chinook Jargon, Ne-matlnomaq, means the "place of matlnomaq" or the (singular) Ne-matlnomag, "the lower river", from the Oregon City Falls toward the Columbia river. Alternatively, Chinookan máɬnumax̣ (also nímaɬnumax̣) ‘those toward water’ (or ‘toward the Columbia River’, known in Chinookan as ímaɬ or wímaɬ ‘the great water’). The explorer William Clark wrote in his Journal: "I entered this river...called Multnomah...from a nation who reside on Wappato Island, a little below the enterence" (quoted from Willamette Landings by H.M. Corning).(see:Portland Basin Chinookan Villages in the early 1800s, Boyd and Zenk,) Although Clark refers to the Willamette River as Multnomah, he may not have understood the meaning. Simply put, Multnomah ("down river" or "toward the great water") is the shortened form of ''nematlnomaq/nímaɬnumax̣".

In 1924, the county's three commissioners were indicted and recalled by voters "in response to 'gross irregularities' in the award of contracts for construction of the Burnside and Ross Island bridges"; since all three had been supported by the Ku Klux Klan, their recall also helped reduce that organization's influence in the city.

Vanport, built north of Portland in 1943 to house workers for Kaiser Shipyards, was destroyed by a flood five years later.

In 1968, the Oregon Legislative Assembly referred a bill, Ballot Measure 5, to voters that would amend the state constitution to allow for consolidated city-county governments when the population is over 300,000. The 1968 voters' pamphlet noted that Multnomah County would be the only county in Oregon affected by the measure and voters approved the referendum in the 1968 general election. Since the approval of Measure 5 in 1968, an initiative to merge the county with Portland has been considered and placed on the county ballot several times.