According to the U.S. Census Bureau, the county has a total area of 557.26 square miles (1,443.3 km), of which 548.38 square miles (1,420.3 km) is land and 8.88 square miles (23.0 km) (1.59%) is water.

State and local protected areas/sites

• Banksia Hall
• Barnwell State Park
• Long Branch Bay Heritage Preserve

Major water bodies

• Georges Creek
• Par Pond
• 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)

Demographics

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. The population density was 41.2 inhabitants per square mile (15.9/km). There were 10,484 housing units at an average density of 19.1 units per square mile (7.4 units/km). 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. In terms of ancestry, 11.5% were American, 5.7% were German, and 5.4% were English.

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.

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.

2000 census

At the 2000 census, 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/km). There were 10,191 housing units at an average density of 19 units per square mile (7.3 units/km). 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.

Government and politics

Prior to 1948, Barnwell County was a Democratic Party stronghold similar to the rest of the Solid South, with Democratic presidential candidates receiving near-unanimous margins of victory in most years. The twenty years from 1948 to 1968 were a highly transitional time for the politics of South Carolina & Barnwell County, largely in part due to the Democratic Party's increasing support for African-American civil rights & enfranchisement. Segregationist candidates Strom Thurmond & George Wallace won the county in those aforementioned years, bookended by Democratic wins in 1952 & 1956 & Republican wins in 1960 & 1964. From 1972 on, the county has primarily Republican, but has become more of a swing county in recent years, backing the national winner in every presidential election from 2000 on except for 2020.

Economy

In 2022, Barnwell County's GDP was $556.3 million (approx. $27,018 per capita). In chained 2017 dollars, its real GDP was $448.3 million (approx. $21,774 per capita). From 2022 through 2024, the unemployment rate for the county has fluctuated from a high of 6.3% in January 2022 and a low of 3.2% in April 2023.

As of April 2024, Walmart is one of the largest employers in the county.

Communities

City

• Barnwell (county seat and largest community)

Towns

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

Notable people

• James Brown (1933–2006), singer
• Rosa Louise Woodberry (1869–1932), journalist and school founder
• Sarah Lowe Twiggs (1839–1920), poet

See also

• List of counties in South Carolina
• National Register of Historic Places listings in Barnwell County, South Carolina