Union is a village in McHenry County, Illinois, United States. The population was 580 at the 2010 census, up from 576 in 2000.
A post office called Union has been in operation since 1852. The village was named for the federal union of the United States.
Union is located at 42°14′7″N 88°32′33″W (42.235237, -88.542379).
According to the 2010 census, Union has a total area of 0.83 square miles (2.15 km2), all land.
At the 2000 census there were 576 people, 204 households, and 158 families living in the village. The population density was 950.4 people per square mile (364.6/km2). There were 208 housing units at an average density of 343.2 per square mile (131.7/km2). The racial makeup of the village was 98.09% White, 0.35% Native American, 0.87% from other races, and 0.69% from two or more races. Hispanic or Latino of any race were 3.99%.
Of the 204 households 37.7% had children under the age of 18 living with them, 63.7% were married couples living together, 9.3% had a female householder with no husband present, and 22.5% were non-families. 15.7% of households were one person and 6.4% were one person aged 65 or older. The average household size was 2.82 and the average family size was 3.15.