Thayer is a village in Sangamon County, Illinois, United States. The population was 693 at the 2010 census. It is part of the Springfield, Illinois Metropolitan Statistical Area.
Thayer is located at 39°32′21″N 89°45′40″W (39.539071, -89.761008).
According to the 2010 census, Thayer has a total area of 0.61 square miles (1.58 km2), all land.
As of the census of 2000, there were 750 people, 276 households, and 203 families residing in the village. The population density was 1,248.4 people per square mile (482.6/km2). There were 288 housing units at an average density of 479.4 per square mile (185.3/km2). The racial makeup of the village was 99.07% White, 0.13% African American, 0.53% from other races, and 0.27% from two or more races. Hispanic or Latino of any race were 0.80% of the population.
There were 276 households out of which 38.4% had children under the age of 18 living with them, 63.4% were married couples living together, 8.3% had a female householder with no husband present, and 26.1% were non-families. 21.4% of all households were made up of individuals and 10.9% had someone living alone who was 65 years of age or older.