Putnam County is the least extensive county in the U.S. state of Illinois. According to the 2010 census, it had a population of 6,006. The county seat is Hennepin. The county was formed in 1825 out of Fulton County and named after Israel Putnam, who was a general in the American Revolution.
According to the U.S. Census Bureau, the county has a total area of 172 square miles (450 km), of which 160 square miles (410 km) is land and 12 square miles (31 km) (7.0%) is water. It is the smallest county in Illinois by area.
As of the 2010 census, there were 6,006 people, 2,509 households, and 1,734 families residing in the county. The population density was 37.5 inhabitants per square mile (14.5/km). There were 3,074 housing units at an average density of 19.2 per square mile (7.4/km). The racial makeup of the county was 96.6% white, 0.5% black or African American, 0.2% Asian, 0.1% American Indian, 1.4% from other races, and 1.1% from two or more races. Those of Hispanic or Latino origin made up 4.2% of the population. In terms of ancestry, 46.5% were German, 16.7% were Irish, 16.3% were Italian, 10.9% were Polish, 10.1% were English, 5.1% were Swedish, and 3.5% were American.
Of the 2,509 households, 27.7% had children under the age of 18 living with them, 57.6% were married couples living together, 7.2% had a female householder with no husband present, 30.9% were non-families, and 26.0% of all households were made up of individuals. The average household size was 2.39 and the average family size was 2.85. The median age was 45.2 years.
The median income for a household in the county was $56,458 and the median income for a family was $68,875. Males had a median income of $50,205 versus $31,886 for females. The per capita income for the county was $27,004. About 6.2% of families and 10.9% of the population were below the poverty line, including 21.8% of those under age 18 and 5.4% of those age 65 or over.