Decatur County is a county located in Northwest Kansas. Its county seat and most populous city is Oberlin. As of the 2020 census, the county population was 2,764. The county was named in honor of Stephen Decatur, Jr., a commodore in the United States Navy who served during both Barbary Wars in North Africa, the Quasi-War with France, and the War of 1812 with Britain.

Oberlin is a city in and the county seat of Decatur County, Kansas, United States. As of the 2020 census, its population was 1,644.