Riley County (standard abbreviation: RL) is a county located in the U.S. state of Kansas. As of the 2010 census, the population was 71,115. The largest city and county seat is Manhattan.
According to the U.S. Census Bureau, the county has a total area of 622 square miles (1,610 km), of which 610 square miles (1,600 km) is land and 12 square miles (31 km) (2.0%) is water.
The eastern border of the county follows the former course of the Big Blue River. The river was dammed in the 1960s and Tuttle Creek Lake was created as a result. The county falls within the Flint Hills region of the state.
Riley County is part of the Manhattan, KS Metropolitan Statistical Area. Millennials (ages 15–34 years old) make up 53.6% of the population of Riley County, one of the highest rates in the United States.
As of the census of 2000, there were 62,843 people, 22,137 households, and 12,263 families residing in the county. The population density was 103 people per square mile (40/km²). There were 23,397 housing units at an average density of 38 per square mile (15/km²). The racial makeup of the county was 84.78% White, 6.88% Black or African American, 0.63% Native American, 3.22% Asian, 0.17% Pacific Islander, 1.89% from other races, and 2.43% from two or more races. 4.57% of the population were Hispanic or Latino of any race.
There were 22,137 households out of which 27.80% had children under the age of 18 living with them, 46.20% were married couples living together, 6.80% had a female householder with no husband present, and 44.60% were non-families. 27.50% of all households were made up of individuals and 6.10% had someone living alone who was 65 years of age or older. The average household size was 2.42 and the average family size was 2.99.
In the county, the population was spread out with 18.80% under the age of 18, 34.50% from 18 to 24, 25.90% from 25 to 44, 13.30% from 45 to 64, and 7.50% who were 65 years of age or older. The median age was 24 years. For every 100 females, there were 114.30 males. For every 100 females age 18 and over, there were 115.40 males.
The median income for a household in the county was $32,042, and the median income for a family was $46,489. Males had a median income of $26,856 versus $23,835 for females. The per capita income for the county was $16,349. About 8.50% of families and 20.60% of the population were below the poverty line, including 11.20% of those under age 18 and 6.70% of those age 65 or over.