Monticello ( /mɒntɨtʃɛloʊ/) is a city in Piatt County, Illinois, United States. The population was 5,138 at the 2000 census, and 5,374 at a 2009 estimate. It is the county seat of Piatt County.
Monticello is located at 40°1′41″N 88°34′23″W / 40.02806°N 88.57306°W / 40.02806; -88.57306 (40.028092, -88.573003).
According to the United States Census Bureau, the city has a total area of 3.0 square miles (7.7 km²), all of it land.