Colchester is a city in McDonough County, Illinois, United States. The population was 1,401 at the 2010 census. The city is named after the town of Colchester, England.
Colchester is located at 40°25′31″N 90°47′32″W (40.425174, -90.792339).
According to the 2010 census, Colchester has a total area of 1 square mile (2.59 km2), all land.
Colchester is the site of Argyle Lake State Park.
Coal was discovered near Colchester in the 1850s.
According to the North American Stratigraphic Code, rock units were given names that included the geographic name of a location where the rock unit was first described. If the rock unit consisted of a dominant rock type, the rock type was included in the name. In this case, when coal was first discovered and described in Colchester, Illinois, the rock unit was named "Colchester Coal". When this particular coal unit was encountered in a different location, the coal was correctly identified as Colchester Coal.
The mines in Colchester attracted immigrants from Pennsylvania. At first these included the descendants of Irish Protestant refugees from the Irish Rebellion of 1798. Later they were joined by Irish Catholic refugees from the Great Famine.