Grundy County is a county located in the U.S. state of Iowa. At the 2020 census, the population was 12,329. The county seat is Grundy Center. The county is named for Felix Grundy, former U.S. Attorney General.

Grundy Center is a city in Grundy County, Iowa. The population was 2,796 at the time of the 2020 census, a 7.7% increase from 2,596 at the 2000 census. Grundy Center is also the county seat of Grundy County. Grundy Center is part of the Waterloo–Cedar Falls Metropolitan Statistical Area.