Washington County, known locally as South County, is a county located in the U.S. state of Rhode Island. As of the 2020 census, the population was 129,839. Rhode Island counties have no governmental functions other than as court administrative boundaries, which are part of the state government.

The area today known as Washington County was part of the ancestral lands of the Narragansett Indian Tribe. During the second half of the 17th Century, about a dozen English colonists from Newport and Massachusetts colonies moved to the area to establish farms; the larger of these were known as "Narragansett Planters."

By the mid-18th century, there were 25 to 30 large plantations in the county, and their owners became very wealthy. The labor for these farms came from enslaved people; it is estimated that about 15% and 25% of Washington County’s population was enslaved.

Washington County was created as Kings County in 1729 within the Colony of Rhode Island and Providence Plantations. It was renamed Washington County on October 29, 1781, in honor of George Washington. At the earliest stage of colonial settlement, the area was called "The Narragansett Country", named after the Naragansett tribe and its tributary tribe the Niantics, both of whom lived in the area.

Early land purchases in the Narragansett Country were effected by settlers after the establishment of Indian trading posts at Fort Neck in Charlestown, and at Smith's Castle in Wickford. A series of conflicts involving the Manisseans on Block Island gave that island to the Massachusetts Bay Colony for a number of years, before being transferred to the Rhode Island Colony under Newport County, and then finally to Washington County in 1959.

The borders of the Narragansett country were disputed for nearly 100 years among the colonies of Rhode Island, Connecticut, and Massachusetts. The Narragansetts had pledged their fealty to King Charles, and the area was known as "The King's Province" and was placed under the authority of Rhode Island "until the King's pleasure was further known". In 1664, a royal commission under Charles II stepped in to adjudicate these conflicting claims. The commission extinguished the claims of Massachusetts, and Rhode Island was granted jurisdiction until the commission finished processing Connecticut's appeals, which were not ended until 1726. Settlements of King's Province were named to reflect the English Restoration, in honor of King Charles II. Towns reflecting this history include the two Kingstowns and Charlestown, as well as the villages of Kingston and West Kingston.

Washington County is also known in Rhode Island as "South County", though some definitions of South County include outside towns, such as East Greenwich in neighboring Kent County, and exclude towns within Washington County, such as New Shoreham.

According to the U.S. Census Bureau, the county has a total area of 563 square miles (1,460 km), of which 329 square miles (850 km) is land and 234 square miles (610 km) (41%) is water. It is the largest county in Rhode Island by total area. The county's topography ranges from flat along the shoreline to gently rolling hills farther inland. The highest point is a large area approximately 560 feet (170 m) above sea level in the Exeter neighborhood of Black Plain; the lowest point is sea level along the coast. The northern boundary west of Davisville is approximately 41.60°N. The western boundary north of Westerly is approximately 71.79°W.

• Block Island National Wildlife Refuge
• John H. Chafee National Wildlife Refuge
• Ninigret National Wildlife Refuge
• Trustom Pond National Wildlife Refuge

• I-95
• US 1
• Route 1A
• Route 2
• Route 3
• Route 4
• Route 78
• Route 91
• Route 102
• Route 108
• Route 110
• Route 112
• Route 138
• Route 165
• Route 216
• Route 402
• Route 403

Historical population

Census	Pop.	Note	%±
1790	18,323		—
1800	16,135		−11.9%
1810	14,962		−7.3%
1820	15,687		4.8%
1830	15,411		−1.8%
1840	14,324		−7.1%
1850	16,430		14.7%
1860	18,715		13.9%
1870	20,097		7.4%
1880	22,495		11.9%
1890	23,649		5.1%
1900	24,154		2.1%
1910	24,942		3.3%
1920	24,932		0.0%
1930	29,334		17.7%
1940	32,493		10.8%
1950	48,542		49.4%
1960	59,054		21.7%
1970	83,586		41.5%
1980	93,317		11.6%
1990	110,006		17.9%
2000	123,546		12.3%
2010	126,979		2.8%
2020	129,839		2.3%
U.S. Decennial Census 1790-1960 1900-1990 1990-2000 2010-2019

As of the 2010 United States Census, there were 126,979 people, 49,177 households, and 32,297 families residing in the county. The population density was 385.7 inhabitants per square mile (148.9/km). There were 62,206 housing units at an average density of 188.9 per square mile (72.9/km). The racial makeup of the county was 93.8% white, 1.6% Asian, 1.2% black or African American, 0.9% American Indian, 0.7% from other races, and 1.8% from two or more races. Those of Hispanic or Latino origin made up 2.4% of the population.

The largest ancestry groups were:

Of the 49,177 households, 28.6% had children under the age of 18 living with them, 52.2% were married couples living together, 9.5% had a female householder with no husband present, 34.3% were non-families, and 26.1% of all households were made up of individuals. The average household size was 2.45 and the average family size was 2.95. The median age was 42.3 years.

The median income for a household in the county was $70,285 and the median income for a family was $87,999. Males had a median income of $59,598 versus $44,851 for females. The per capita income for the county was $34,737. About 3.4% of families and 7.4% of the population were below the poverty line, including 6.3% of those under age 18 and 5.0% of those age 65 or over.

• Charlestown
• Exeter
• Hopkinton
• Narragansett
• New Shoreham
• North Kingstown
• Richmond
• South Kingstown (traditional county seat)
• Westerly

Similar to other counties in Rhode Island, Washington County votes Democratic in presidential elections, having last voted Republican in 1984.

United States presidential election results for Washington County, Rhode Island

Year	Republican / Whig		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2020	29,818	39.20%	44,549	58.57%	1,693	2.23%
2016	27,230	41.03%	33,741	50.84%	5,398	8.13%
2012	25,366	40.34%	35,888	57.07%	1,625	2.58%
2008	25,624	38.73%	39,082	59.07%	1,454	2.20%
2004	26,533	42.36%	34,679	55.37%	1,422	2.27%
2000	21,253	37.80%	29,560	52.58%	5,411	9.62%
1996	16,302	32.71%	25,958	52.09%	7,572	15.20%
1992	16,211	30.62%	23,009	43.46%	13,724	25.92%
1988	21,650	48.04%	23,210	51.51%	202	0.45%
1984	24,365	57.59%	17,793	42.06%	147	0.35%
1980	16,932	41.47%	16,429	40.24%	7,466	18.29%
1976	17,856	49.57%	17,980	49.91%	186	0.52%
1972	19,280	58.46%	13,637	41.35%	62	0.19%
1968	11,639	43.40%	13,851	51.65%	1,328	4.95%
1964	7,342	29.63%	17,434	70.37%	0	0.00%
1960	12,651	52.21%	11,580	47.79%	0	0.00%
1956	14,278	65.71%	7,450	34.29%	0	0.00%
1952	13,389	61.29%	8,448	38.67%	9	0.04%
1948	9,522	55.88%	7,379	43.30%	140	0.82%
1944	8,233	56.12%	6,419	43.76%	18	0.12%
1940	9,233	60.54%	6,001	39.35%	17	0.11%
1936	8,764	58.21%	5,956	39.56%	336	2.23%
1932	7,307	58.42%	5,047	40.35%	153	1.22%
1928	7,793	68.84%	3,500	30.92%	27	0.24%
1924	8,038	75.21%	2,366	22.14%	283	2.65%
1920	6,420	74.93%	2,012	23.48%	136	1.59%
1916	2,837	55.17%	2,224	43.25%	81	1.58%
1912	2,129	44.91%	1,691	35.67%	921	19.43%
1908	3,043	66.46%	1,278	27.91%	258	5.63%
1904	3,189	70.63%	1,197	26.51%	129	2.86%
1900	2,421	66.35%	960	26.31%	268	7.34%
1896	3,040	72.05%	654	15.50%	525	12.44%
1892	2,183	52.83%	1,501	36.33%	448	10.84%
1888	2,346	57.54%	1,492	36.60%	239	5.86%
1884	2,014	60.08%	1,155	34.46%	183	5.46%
1880	2,017	62.02%	1,229	37.79%	6	0.18%
1852	1,022	46.71%	1,086	49.63%	80	3.66%
1844	967	57.59%	712	42.41%	0	0.00%

• National Register of Historic Places listings in Washington County, Rhode Island

41°23′N 71°37′W / 41.39°N 71.62°W / 41.39; -71.62