According to the U.S. Census Bureau, the county has a total area of, of which is land and is water.
Climate and weather
In recent years, average temperatures in the county seat of Newton have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1994 and a record high of was recorded in July 1954. Average monthly precipitation ranged from in January to in May.
As of the 2010 United States Census, there were 9,698 people, 3,940 households, and 2,800 families living in the county. The population density was. There were 4,345 housing units at an average density of. The racial makeup of the county was 98.6% white, 0.2% Asian, 0.1% American Indian, 0.1% black or African American, 0.3% from other races, and 0.6% from two or more races. Those of Hispanic or Latino origin made up 0.8% of the population. In terms of ancestry, 39.5% were German, 12.5% were American, 11.0% were Irish, and 9.1% were English. Of the 3,940 households, 29.7% had children under the age of 18 living with them, 59.1% were married couples living together, 7.9% had a female householder with no husband present, 28.9% were non-families, and 24.5% of all households were made up of individuals. The average household size was 2.45 and the average family size was 2.90. The median age was 42.7 years. The median income for a household in the county was $46,546 and the median income for a family was $53,034. Males had a median income of $39,167 versus $24,856 for females. The per capita income for the county was $21,467. About 6.3% of families and 8.5% of the population were below the poverty line, including 10.6% of those under age 18 and 5.1% of those aged 65 or over.
Jasper is politically a fairly typical "anti-Yankee" Southern Illinois county. Opposition to the "Yankee" Republican Party and that party's Civil War meant that Jasper County voted solidly Democratic until isolationist sentiment drove its voters to Warren G. Harding in 1920. Since the New Deal, the county has shown a steady trend away from the Democratic Party due to major shifts in that party's views – initially on economic policies, and since the 1990s on social issues. Only one Democrat – Lyndon Johnson in his 1964 landslide – has won a majority since 1940, and as is typical of the Upland South, Barack Obama in 2012 and Hillary Clinton did far worse than any previous Democrat.
