Sully County, South Dakota

Sully County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 1,446, making it the fifth-least populous county in South Dakota. Its county seat is Onida. The county was created in 1873 and organized in 1883. It is named after General Alfred Sully, who built Fort Sully.

Sully County is included in the Micropolitan Statistical Area of Pierre.

Sully County was the location of the largest African American homesteader settlement in the state, the Blair Colony. An Illinois man named Norvel Blair arrived in Fairbank Township in 1884, after sending his sons Benjamin and Patrick to investigate the area for settlement. Blair's financial success, achieved through farming and breeding racehorses, attracted dozens of other Black families to the colony. Blair became the first Black South Dakotan to serve on a school board.

The decline of South Dakota agriculture during the Great Depression led most Blair Colony residents to seek work in larger cities like Minneapolis and Chicago. A historical marker in the county seat of Onida remembers the colony.

Geography

The west boundary line of Sully County is defined by the meanderings of the Missouri River, which flows southward along its edge. The county's terrain is composed of semi-arid rolling hills, partially devoted to agriculture. The terrain slopes to the south and east, but the west portion of the county slopes westward into the river valley. The county's highest point is along the midpoint of its north boundary line, at ASL. The county has a total area of , of which is land and (5.9%) is water.

The eastern portion of South Dakota's counties (48 of 66) observe Central Time; the western counties (18 of 66) observe Mountain Time. Sully County is at the western edge of those counties that observe Central Time.

Major highways

20px U.S. Highway 83
23px South Dakota Highway 1804

Adjacent counties

Potter County – north
Hyde County – east
Hughes County – south
Stanley County – southwest (observes mountain time)
Dewey County – northwest (observes Mountain Time)

Protected areas

Source:

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|footnote=U.S. Decennial Census<br>1790–1960 1900–1990<br>1990–2000 2010–2020 The population density was .

Of the residents, 21.2% were under the age of 18 and 22.2% were 65 years of age or older; the median age was 45.4 years. For every 100 females there were 112.0 males, and for every 100 females age 18 and over there were 111.1 males.

The racial makeup of the county was 93.4% White, 0.3% Black or African American, 0.7% American Indian and Alaska Native, 0.4% Asian, 0.6% from some other race, and 4.6% from two or more races. Hispanic or Latino residents of any race comprised 2.9% of the population.

Notable person

Irwin Gunsalus (1912–2008), biochemist, was born in Sully County.