Sycamore is a city in DeKalb County, Illinois, United States. It has a commercial district based and centered on Illinois Route 64. The population was 17,519 at the 2010 census, up from 12,020 in 2000. As of 2019, the population has grown to an estimated 18,322. Sycamore is the county seat of DeKalb County and was named after the sycamore tree.