Milwaukee County (/mɪlˈwɔːki/ ^ⓘ) is a county in the U.S. state of Wisconsin. The population was 939,489 at the 2020 census. It is both the most populous and most densely populated county in Wisconsin, containing about 15% of the state's population; it is also the 62nd-most populous county nationwide. The county seat is Milwaukee, the most populous city in Wisconsin. Named after the Milwaukee River, the county was created in 1834 as part of Michigan Territory and organized the following year. Milwaukee County is the most populous county of the Milwaukee metropolitan area, as well as of the Milwaukee–Racine–Waukesha combined statistical area.

Uniquely among Wisconsin counties, Milwaukee County is completely incorporated (i.e., no part of the county has the unincorporated "town" jurisdiction). There are 19 municipalities in Milwaukee County; 10 incorporated as cities and 9 incorporated as villages. After the city of Milwaukee, the most populous in 2020 were West Allis (60,325), Wauwatosa (48,387), Greenfield (37,803), Oak Creek (36,497), and Franklin (36,816). The county is home to two major-league professional sports teams, the Milwaukee Bucks and Milwaukee Brewers, and the world's largest music festival, Summerfest.

Portions of what is now Milwaukee County are known to have been inhabited by a number of Native American tribes, including the Sauk, Meskwaki or "Fox", Menomonee, Ojibwe and Potawotami, with elements of other tribes attested as well.

In 1818, when the land later to be Wisconsin was made part of Michigan Territory, territorial governor Lewis Cass created Brown County, which at that time included all the land now part of Milwaukee County. It remained a part of Brown county until 1834, when Milwaukee County was created, including the area south of the line between townships eleven and twelve north (i.e., the northern boundary of Washington and Ozaukee counties), west of Lake Michigan, north of Illinois, and east of the line which now separates Green and Rock counties. This territory encompassed all of what are now Milwaukee, Jefferson, Kenosha, Ozaukee, Racine, Rock, Walworth, Washington, and Waukesha counties, as well as large parts of the present-day Columbia, Dane and Dodge counties.

Milwaukee County remained attached to Brown County for judicial purposes until August 25, 1835, when an act was passed by the Michigan territorial legislature giving it an independent organization. In 1836, the legislature divided the area south and east of the Wisconsin and Fox rivers into counties, as a consequence reducing Milwaukee County's extent to what is now Milwaukee and Waukesha counties. In 1846 Waukesha County was created by taking from Milwaukee all of the territory west of range 21, reducing Milwaukee County to its present boundaries.

The county peaked in its relative importance in Wisconsin in the 1930s–1960s, when about 25% of the state's population resided in Milwaukee County. Its population has been shrinking since 1970.

According to the U.S. Census Bureau, the county has a total area of 1,189 square miles (3,080 km²), of which 241 square miles (620 km²) is land and 948 square miles (2,460 km²) (80%) is water. It is the third-smallest county in Wisconsin by land area. It is watered by the Milwaukee, Menomonee, Kinnickinnic, and Root Rivers. The surface is undulating, and the soil calcareous and fertile.

As of the census of 2020, the population was 939,489. The population density was 3,890.5 people per square mile (1,502.1 people/km²). There were 424,191 housing units at an average density of 1,756.6 units per square mile (678.2 units/km²). The racial makeup of the county (including Hispanics in the racial counts) was 52.0% White, 26.2% Black or African American, 4.9% Asian, 0.8% Native American, 6.8% from other races, and 9.3% from two or more races. Ethnically, the population was 16.3% Hispanic or Latino of any race.