As of the census of 2000, there were 20,678 people, 7,042 households, and 5,091 families residing in the county. The population density was 32 people per square mile (12/km²). There were 7,928 housing units at an average density of 12 per square mile (5/km²). The racial makeup of the county was 42.39% White, 52.69% Black or African American, 0.37% Native American, 0.44% Asian, 0.05% Pacific Islander, 3.39% from other races, and 0.67% from two or more races. 5.75% of the population were Hispanic or Latino of any race.
There were 7,042 households out of which 34.5% had children under the age of 18 living with them, 48.1% were married couples living together, 18.2% had a female householder with no husband present, and 27.7% were non-families. 23.2% of all households were made up of individuals and 8.8% had someone living alone who was 65 years of age or older. The average household size was 2.75 and the average family size was 3.22.
In the county, the population was spread out with 26.8% under the age of 18, 10.3% from 18 to 24, 30.7% from 25 to 44, 21.2% from 45 to 64, and 11.0% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 111.0 males. For every 100 females age 18 and over, there were 111.3 males.
The median income for a household in the county was $30,727, and the median income for a family was $36,793. Males had a median income of $29,407 versus $21,055 for females. The per capita income for the county was $14,161. About 15.4% of families and 20.7% of the population were below the poverty line, including 26.3% of those under age 18 and 21.4% of those age 65 or over.
Jasper county is largely rural, with only about 18% of the population residing in the Ridgeland Urban Cluster (2000 pop.: 3,585). However, the region of the county near Bluffton and Hilton Island Island (towns within Beaufort County) is growing in population, and now Jasper County is included within the Hilton Head Island-Beaufort Micropolitan Area, as classified by the U.S. Census Bureau.