() is a city is located primarily in Washington County in the U.S. state of Oregon. A small portion of the city is also located in neighboring Clackamas County. It is a southwestern suburb in the Portland Metropolitan Area that is located south of Tigard. The population was 22,791 at the 2000 census. The 2006 estimate is 25,650 residents.
The name of the city is taken from the Tualatin River, which flows along most of the city's northern boundary. According to
, a post office with the spelling "Tualitin" was established November 5, 1869, and the spelling changed to "Tualatin" in 1915. In the 1880s, the settlement was sometimes called Bridgeport, probably because of the nearby bridge, one of the first built over the Tualatin River.
In 1962, a fossilized mastodon was excavated in what is now the Fred Meyer parking lot.