Decatur County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 7,645. The county seat is Leon. This county is named for Stephen Decatur Jr., a hero in the War of 1812.

Lamoni is a city in Decatur County, Iowa, United States. The population was 1,969 at the time of the 2020 Census. Lamoni is the home of Graceland University, affiliated with the Community of Christ, a denomination of the Latter Day Saint movement. The city was the church's headquarters from 1880 to 1920, after which it moved to Independence, Missouri.