Nez Perce County was originally organized in 1861, when the area was part of Washington Territory. It was reorganized in 1864 by the Idaho Territorial Legislature.
Nez Perce County is part of the Lewiston, ID-WA Metropolitan Statistical Area, which includes all of both Nez Perce County in Idaho and Asotin County in Washington.
As of the census of 2000, there were 37,410 people, 15,286 households, and 10,149 families residing in the county. The population density was 44 people per square mile (17/km²). There were 16,203 housing units at an average density of 19 per square mile (7/km²). The racial makeup of the county was 91.58% White, 0.28% Black or African American, 5.31% Native American, 0.65% Asian, 0.07% Pacific Islander, 0.50% from other races, and 1.60% from two or more races. 1.93% of the population were Hispanic or Latino of any race. 25.1% were of German, 11.3% English, 11.0% American, 8.9% Irish and 5.6% Norwegian ancestry according to Census 2000.
There were 15,286 households out of which 29.00% had children under the age of 18 living with them, 52.80% were married couples living together, 9.30% had a female householder with no husband present, and 33.60% were non-families. 26.70% of all households were made up of individuals and 11.30% had someone living alone who was 65 years of age or older. The average household size was 2.40 and the average family size was 2.90.
In the county the population was spread out with 23.80% under the age of 18, 10.00% from 18 to 24, 26.70% from 25 to 44, 23.00% from 45 to 64, and 16.50% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 96.70 males. For every 100 females age 18 and over, there were 93.80 males.
The median income for a household in the county was $36,282, and the median income for a family was $44,212. Males had a median income of $34,688 versus $23,014 for females. The per capita income for the county was $18,544. About 8.60% of families and 12.20% of the population were below the poverty line, including 15.40% of those under age 18 and 6.70% of those age 65 or over.