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.

In 1854, the Kansas Territory was organized, then in 1861 Kansas became the 34th U.S. state.

Decatur County was established by an act of the state legislature on 1873-03-20, which defined the original borders of the county as:

Commencing where the east line of range 26 west, intersects the fortieth degree of north latitude; thence south, with the range line, to the first standard parallel; thence west with said parallel to the east line of range 31 west; thence north with said range line to the fortieth degree of north latitude; thence east with said parallel to the place of beginning.

The county had been surveyed the previous year, and its first settlers had been arriving since then.

Independent county government was established on 1879-12-11, by proclamation of Governor John A. Martin in response to a petition. Martin originally proclaimed Oberlin as the county seat, as an initial temporary arrangement. It was chosen as the permanent seat by an election on 1880-02-03.

The county is named for the Navy war hero Commodore Stephen Decatur, Jr., who served during the First Barbary War and the Second Barbary War and the War of 1812.

In 1878, the Sappa Creek valley in Decatur county was the scene of the last raid by Native Americans (Indians) in Kansas. In the Northern Cheyenne Exodus after the Battle of Punished Woman's Fork, a band of Cheyenne needing horses and provisions raged through the valley, killing more than 30 civilians and raping several woman. Several Cheyenne elderly, women, and children were also killed in the region by soldiers and civilians. In Oberlin, the Decatur County Last Indian Raid Museum commemorates the Cheyenne raid.