Zip Code 57078 Description. Zip Code 57078 is located in the state of South Dakota in the Sioux Falls (Mitchell) metro area. Zip code 57078 is primarily located in Yankton County. The official US Postal Service name for 57078 is YANKTON, South Dakota. Portions of zip code 57078 are contained within or border the city limits of Yankton, SD, . The area code for zip code 57078 is 605.