Ozaukee County is a county in the U.S. state of Wisconsin. As of the 2020 census, the population was 91,503. Its county seat is Port Washington. Ozaukee County is included in the Milwaukee metropolitan area.

As of the 2000 Census, Ozaukee County had the second-lowest poverty rate of any county in the United States, at 2.6%. In terms of per capita income, it is the 25th-wealthiest county in the country.

"Ozaukee" comes from the Ojibwe name for the Sauk people. It probably means "people living at the mouth of a river."

The Hilgen Spring Mound Site is one of the oldest-known sites of human habitation of Ozaukee County. Located near Cedar Creek in the eastern part of the City of Cedarburg, the site consists of three conical burial mounds constructed by early Woodland period Mound Builders. In 1968, archaeologists from the Milwaukee Public Museum found human burials and artifacts, including stone altars, arrowheads, and pottery shards, during an excavation of one of the mounds. Radiocarbon samples from the excavation date the mounds' construction to approximately 480 BCE, making it one of the oldest mound groups in the state.

In the mid-1800s, Increase A. Lapham identified a group of circular mounds in the Saukville area and found a stone ax. In his writing, Lapham did not speculate about the age of the artifact or the mounds. An additional artifact of the early Native American presence in the Saukville area is the Ozaukee County Birdstone, discovered by a six-year-old farm boy in 1891. While the exact age of the Ozaukee County Birdstone remains uncertain, many birdstones date from a period ranging from 3000 BCE to 500 BCE.

In the early 19th century, the Native Americans living in Ozaukee County included the Menominee, Potawatomi, and Sauk people. There were numerous Native American villages in the county along the Milwaukee River and its tributaries. The Menominee surrendered their claims to the land east of the Milwaukee River to the United States Federal Government in 1832 through the Treaty of Washington. The Potawatomi surrendered their claims to the land west of the river in 1833 through the 1833 Treaty of Chicago, which (after being ratified in 1835) required them to leave the area by 1838. While many Potawatomi people moved west of the Mississippi River to Kansas, some chose to remain in Wisconsin, and were known as "strolling Potawatomi" because they were migrant squatters. Eventually the Potawatomi who evaded forced removal gathered in northern Wisconsin, where they formed the Forest County Potawatomi Community.

The first whites in the area were primarily New England land speculators, who began purchasing land from the government in 1835 at the price of $1.25 per acre. One of these land speculators was Wooster Harrison, who settled the land that would become Port Washington in 1835, which he originally named "Wisconsin City." At the time, the land was part of Washington County, and there were proposals that Port Washington become the county seat. However, Port Washington was far from the county's other early settlements, including Mequon, Grafton and Germantown. In 1850, the Wisconsin legislature bisected Washington County into northern and southern counties, with Port Washington as the northern seat and Cedarburg as the southern. County residents failed to ratify the bill, and in 1853 the legislature instead bisected the county into eastern and western sections, creating Ozaukee County. Port Washington became the seat of the new county, and the Washington County seat moved to West Bend.

In the 1840s, German, Irish, and Luxembourger immigrants began settling in the county. Germans were the largest ethnic group in and 19th century Ozaukee County, with seven in eight residents being of German descent according to the 1870 census. The earliest settlements formed around grist- and sawmills located on the county's waterways. Cedarburg, Grafton, Hamilton, Newburg, Saukville, and Thiensville all had mills by end of the 1840s. In the 19th and early 20th centuries, the county economy was primarily based on agriculture.