Yankton County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 23,310. Its county seat is Yankton. Yankton County comprises the Yankton, SD Micropolitan Statistical Area.
 Yankton County lies on the south side of South Dakota. Its south boundary line abuts the north boundary line of the state of Nebraska (across the Missouri River). The Missouri flows eastward along the county's south border. The James River flows south-southeastward through the west central portion of the county, discharging into the Missouri near the midpoint of the county's south line. The county terrain consists of rolling hills, carved by creeks and drainages, hosting several lakes and ponds. The area is devoted to agriculture. The terrain slopes to the south and the east. Its highest point is 1,473 ft (449 m) ASL, on the eastern portion of its north boundary line. The county has a total area of 532 square miles (1,380 km2), of which 521 square miles (1,350 km2) is land and 11 square miles (28 km2) (2.1%) is water.
 Lewis & Clark Lake, a popular regional tourist and recreational destination is found in the southwestern part of the county.
 As of the census of 2020, there were 23,310 people.
 As of the census of 2010, there were 22,438 people, 8,770 households, and 5,476 families in the county. The population density was 43.1 inhabitants per square mile (16.6/km2). There were 9,652 housing units at an average density of 18.5 per square mile (7.1/km2). The racial makeup of the county was 92.8% white, 2.5% American Indian, 1.5% black or African American, 0.5% Asian, 1.1% from other races, and 1.4% from two or more races. Those of Hispanic or Latino origin made up 2.7% of the population. In terms of ancestry.
 Of the 8,770 households, 29.2% had children under the age of 18 living with them, 50.1% were married couples living together, 8.8% had a female householder with no husband present, 37.6% were non-families, and 32.0% of all households were made up of individuals. The average household size was 2.30 and the average family size was 2.90. The median age was 41.3 years.
 The median income for a household in the county was $47,124 and the median income for a family was $62,070. Males had a median income of $37,637 versus $29,488 for females. The per capita income for the county was $24,776. About 5.0% of families and 11.2% of the population were below the poverty line, including 10.7% of those under age 18 and 13.5% of those age 65 or over.
 Jamesville Colony
 Yankton County is Governed by the County Commission, a five-member legislative body elected at-large. The current commissioners are:
 The county is located in South Dakota Legislative District 18. As of 2019, Yankton County is represented in the South Dakota Senate by Senator Craig Kennedy-(D), and in the South Dakota House of Representatives by State Representatives Jean Hunhoff-(R) and Ryan Cwach-(D). The county is located in South Dakota's At-Large Congressional District, currently held by Republican Dusty Johnson.
 In national elections, Yankton County voters have been reliably Republican for several decades. In no national election since 1964 has the county selected the Democratic Party candidate.
 The Yankton County Sheriff's Office provides law enforcement in the county and operates the 120-bed county jail. As of 2021 the current Sheriff is James "Jim" C. Vlahakis.
 Lewis and Clark Lake is located in southwestern Yankton County, four miles (6.4 km) west of Yankton. The Lewis & Clark Lake area is a popular regional tourist and recreation destination with parks, trails, campgrounds, boat ramps, and marinas. Gavins Point Dam, which creates Lewis & Clark Lake, is a popular fishing destination on the Missouri River. The South Dakota Department of Game, Fish, and Parks operates the Lewis & Clark State Recreation Area on the northern shore of the lake.
 Below Gavins Point Dam, the Missouri National Recreational River is home to one of the few non-channelized portions of the "Big Muddy", and is popular with birdwatchers, hunters, canoers, and kayakers.
 43°01′N 97°23′W﻿ / ﻿43.01°N 97.39°W﻿ / 43.01; -97.39