Colchester is a city in McDonough County, Illinois, United States. The population was 1,493 at the 2000 census. The city is named after the provincial town of Colchester, England.
Colchester is located at 40°25′31″N 90°47′32″W / 40.42528°N 90.79222°W / 40.42528; -90.79222 (40.425174, -90.792339).
According to the United States Census Bureau, the city has a total area of 1.0 square miles (2.6 km²), all of it land.