Tualatin () is a city 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 26,054 at the 2010 census.
The name of the city is taken from the Tualatin River, which flows along most of the city's northern boundary. It is probably a Native American word meaning "lazy" or "sluggish" but possibly meaning "treeless plain" for the plain near the river or "forked" for its many tributaries. According to Oregon Geographic Names, a post office with the spelling "Tualitin" was established November 5, 1869, and the spelling changed to "Tualatin" in 1915.
In the 1850s, the settlement was first called Galbreath after its founder Samuel Galbreath. In 1853, Galbreath built the first bridge over the Tualatin river, and the town became known as Bridgeport. In the 1880s, John Sweek platted a town around the new railroad depot, and named the town Tualatin.