Olmsted County is a county in the U.S. state of Minnesota. As of the 2010 census, the population was 144,248, and has an estimated population of 156,277 as of 2018. Its county seat and largest city is Rochester.
Olmsted County is drained by three rivers, all flowing to the Mississippi. The Zumbro River flows northward through the west central part of the county, into Wabasha County. The Whitewater flows northeast from the northeast part of the county into Winona County, and the Root flows east-southeastward through the lower part of the county into Fillmore County. The county terrain consists of low rolling hills, etched by drainage gullies and marked by occasional buttes. The available area is devoted to agriculture or is developed for other productive uses. The county terrain slopes to the east and north, and its highest point is a hill 7.5 miles (12 km) west of Stewartville, at 1,380′ (421m) ASL. The county has a total area of 655 square miles (1,700 km), of which 653 square miles (1,690 km) is land and 1.5 square miles (3.9 km) (0.2%) is water.
It is one of four counties in Minnesota that does not have any natural lakes (the other three being Mower, Pipestone, and Rock).
As of the 2000 United States Census, there were 124,277 people, 47,807 households, and 32,317 families in the county. The population density was 190/sqmi (73.5/km²). There were 49,422 housing units at an average density of 75.7/sqmi (29.2/km²). The racial makeup of the county was 90.33% White, 2.68% Black or African American, 0.26% Native American, 4.27% Asian, 0.03% Pacific Islander, 0.92% from other races, and 1.51% from two or more races. 2.38% of the population were Hispanic or Latino of any race.
There were 47,807 households out of which 35.20% had children under the age of 18 living with them, 56.70% were married couples living together, 8.00% had a female householder with no husband present, and 32.40% were non-families. 25.80% of all households were made up of individuals and 7.60% had someone living alone who was 65 years of age or older. The average household size was 2.53 and the average family size was 3.09.
The county population contained 27.00% under the age of 18, 8.50% from 18 to 24, 32.20% from 25 to 44, 21.60% from 45 to 64, and 10.80% who were 65 years of age or older. The median age was 35 years. For every 100 females, there were 96.60 males. For every 100 females age 18 and over, there were 93.60 males.
The median income for a household in the county was $51,316, and the median income for a family was $61,610. Males had a median income of $40,196 versus $29,994 for females. The per capita income for the county was $24,939. About 3.80% of families and 6.40% of the population were below the poverty line, including 6.70% of those under age 18 and 9.50% of those age 65 or over.