High Quality Content by WIKIPEDIA articles! Jefferson County (county code JF) is a county located in Northeast Kansas, in the Central United States. As of the 2010 census, the county population was 19,126. Its county seat is Oskaloosa, and its most populous city is Valley Falls. The county along with Shawnee, Jackson, Osage, and Wabaunsee counties is included in the Topeka, Kansas, Metropolitan Statistical Area.