Grundy County is a county located in the U.S. state of Missouri. As of the 2020 census, the population was 9,808. Its county seat is Trenton. The county was organized January 2, 1841, from part of Livingston County, Missouri and named after U.S. Attorney General Felix Grundy.

Trenton is a city in Grundy County, Missouri, United States. The population was 5,609 at the 2020 census. It is the county seat of Grundy County. The city used to be the world's largest producer of vienna sausages.