State and local protected areas/sites

• Banksia Hall
• Barnwell State Park
• Long Branch Bay Heritage Preserve^[7]

Major water bodies

• Georges Creek^[8]
• Par Pond^[9]
• Salkehatchie River
• Savannah River
• South Fork Edisto River

Adjacent counties

• Aiken County – north
• Bamberg County – east
• Orangeburg County – east
• Allendale County – southeast
• Burke County, Georgia – southwest

Major highways

• US 78
• 
  US 78 Bus.
• US 278
• SC 3
• SC 37
• SC 39
• SC 64
• SC 70
• SC 125
• SC 300
• SC 304

Major infrastructure

• Barnwell Regional Airport
• Savannah River Site (part)

2020 census

As of the 2020 census, there were 20,589 people, 8,360 households, and 5,260 families residing in the county.

2010 census

At the 2010 census, there were 22,621 people, 8,937 households, and 6,055 families living in the county.^[16][14] The population density was 41.2 inhabitants per square mile (15.9/km²). There were 10,484 housing units at an average density of 19.1 units per square mile (7.4 units/km²).^[17] The racial makeup of the county was 52.6% white, 44.3% black or African American, 0.6% Asian, 0.4% American Indian, 0.7% from other races, and 1.5% from two or more races. Those of Hispanic or Latino origin made up 1.8% of the population.^[16] In terms of ancestry, 11.5% were American, 5.7% were German, and 5.4% were English.^[18]

Of the 8,937 households, 34.0% had children under the age of 18 living with them, 41.8% were married couples living together, 20.8% had a female householder with no husband present, 32.2% were non-families, and 28.4% of all households were made up of individuals. The average household size was 2.50 and the average family size was 3.05. The median age was 38.8 years.^[16]

The median income for a household in the county was $33,816 and the median income for a family was $41,764. Males had a median income of $35,957 versus $30,291 for females. The per capita income for the county was $17,592. About 20.8% of families and 25.4% of the population were below the poverty line, including 39.6% of those under age 18 and 11.5% of those age 65 or over.^[19]

2000 census

At the 2000 census,^[20] there were 23,478 people, 9,021 households, and 6,431 families living in the county. The population density was 43 people per square mile (17 people/km²). There were 10,191 housing units at an average density of 19 units per square mile (7.3 units/km²). The racial makeup of the county was 55.18% White, 42.55% Black or African American, 0.35% Native American, 0.39% Asian, 0.03% Pacific Islander, 0.78% from other races, and 0.72% from two or more races. 1.39% of the population were Hispanic or Latino of any race.

There were 9,021 households, out of which 34.80% had children under the age of 18 living with them, 47.40% were married couples living together, 19.30% had a female householder with no husband present, and 28.70% were non-families. 25.60% of all households were made up of individuals, and 10.20% had someone living alone who was 65 years of age or older. The average household size was 2.57 and the average family size was 3.08.

In the county, the population was spread out, with 28.10% under the age of 18, 8.70% from 18 to 24, 27.90% from 25 to 44, 22.60% from 45 to 64, and 12.60% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 92.70 males. For every 100 females age 18 and over, there were 86.50 males.

The median income for a household in the county was $28,591, and the median income for a family was $35,866. Males had a median income of $31,161 versus $21,904 for females. The per capita income for the county was $15,870. About 17.90% of families and 20.90% of the population were below the poverty line, including 27.30% of those under age 18 and 24.40% of those age 65 or over.

City

• Barnwell (county seat and largest community)

Towns

• Blackville
• Elko
• Hilda
• Kline
• Snelling
• Williston