Keene is a city in Cheshire County, New Hampshire, United States. The population was 23,047 at the 2020 census, down from 23,409 at the …Keene is a city in Cheshire County, New Hampshire, United States. The population was 23,047 at the 2020 census, down from 23,409 at the 2010 census. It is the county seat and the only city in the county.