Jefferson County was formed out of Thurston County on December 22, 1852, by the legislature of Oregon Territory, and included the northern 4,854 square miles (12,571 km²) portion of the Olympic Peninsula. On April 26, 1854, the legislature of Washington Territory created Clallam County from the northwestern 2,670 square miles (6,916 km²) portion of this original area.
The Hood Canal Bridge connects Jefferson County to Kitsap County, Washington. The Keystone-Port Townsend route of the Washington State Ferries connects the county to Whidbey Island in Island County, Washington.
As of the census of 2000, there were 25,953 people, 11,645 households, and 7,580 families residing in the county. The population density was 14 people per square mile (6/km²). There were 14,144 housing units at an average density of 8 per square mile (3/km²). The racial makeup of the county was 92.17% White, 0.42% Black or African American, 2.31% Native American, 1.19% Asian, 0.13% Pacific Islander, 0.76% from other races, and 3.02% from two or more races. 2.06% of the population were Hispanic or Latino of any race. 17.4% were of German, 14.8% English, 9.9% Irish and 7.5% Norwegian ancestry according to Census 2000. 97.1% spoke English and 1.0% Spanish as their first language.
There were 11,645 households out of which 23.20% had children under the age of 18 living with them, 53.60% were married couples living together, 8.20% had a female householder with no husband present, and 34.90% were non-families. 28.50% of all households were made up of individuals and 11.70% had someone living alone who was 65 years of age or older. The average household size was 2.21 and the average family size was 2.67.
In the county, the population was spread out with 19.80% under the age of 18, 5.00% from 18 to 24, 21.60% from 25 to 44, 32.50% from 45 to 64, and 21.10% who were 65 years of age or older. The median age was 47 years. For every 100 females there were 95.80 males. For every 100 females age 18 and over, there were 94.40 males.
The median income for a household in the county was $37,869, and the median income for a family was $45,415. Males had a median income of $37,210 versus $25,831 for females. The per capita income for the county was $22,211. About 7.20% of families and 11.30% of the population were below the poverty line, including 16.60% of those under age 18 and 6.00% of those age 65 or over.