Jefferson is a city in, and the county seat of Greene County, Iowa, United States, along the North Raccoon River. The population was 4,182 at the time of the 2020 census. It is the home of the Mahanay Memorial Bell Tower, 168 feet 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. A new U.S. Route 30 is located approximately one mile to the north.