As of the U.S. Census of 2000, Seattle had a population of 563,374 (573,911 as of 2005) and in all the Greater Puget Sound metropolitan area is home to almost 3.8 million people. The population today is approximately 73.40% white, one of the highest percentages of whites for a major American city. The city also has one of the nation's highest percentages of multiracial ancestry: 4.70% claim ancestry from two or more races. According to the 2000 U.S. Census, 13.71% of Seattleites are Asian Americans, 8.44% are African Americans, 1.10% are Native Americans, 0.50% are Pacific Islanders, and 6.84% are from other backgrounds. 5.28% of the population is Hispanic or Latino of any race. Seattle's robust economy and multi-cultural backgrounds has attracted immigrants from all over the world.

The median income for a household in the city is $45,736, and the median income for a family is $62,195. Males have a median income of $40,929 versus $35,134 for females. The per capita income for the city is $30,306. 11.8% of the population and 6.9% of families are below the poverty line. Out of the total people living in poverty, 13.8% are under the age of 18 and 10.2% are 65 or older.

Seattle has seen a major increase in legal and illegal immigration in recent decades. The foreign-born population increased 40% between the 1990 and 2000 censuses. Hispanics are believed to be the most rapidly growing ethnic group in Washington, with an estimated increase of 10% just in the years 2000 to 2002, though they have tended to settle outside the city, in rural areas where agricultural jobs are abundant.

It is estimated that 1.25% of the population is homeless, and that up to 14% of Seattle's homeless are children and young adults. Seattle non-profit organizations dealing with poverty and related issues include the Fremont Public Association, the Asian Counseling and Referral Service, and the Seattle Indian Center. In September 2005, King County adopted a "Ten-Year Plan to End Homelessness", one of whose near-term results is a shift of funding from homeless shelter beds to permanent housing.

In 2005, Men's Fitness magazine named Seattle the fittest city in the U.S.