Jefferson is a city in Greene County, Iowa, United States, along the North Raccoon River. The population was 4,345 at the 2010 census. It is the county seat of Greene County. It is the home of the Mahanay Memorial Bell Tower, 168 feet (51 m) tall, located on the town square, and visible for miles. The tower is named for Floyd Mahanay, a businessman, philanthropist, and former resident. Jefferson is bisected east to west by the old Lincoln Highway (formerly U.S. 30). A new U.S. Route 30 is located approximately one mile to the north.
While platted and settled a few years earlier, "New Jefferson" was organized in the winter of 1855–56 and officially incorporated in January 1872. It began as a farming community and remains so today. The first settlers of the new city were the family of George S. Walton who built there in 1855.