Mequon () is the most populous city in Ozaukee County, Wisconsin, United States. The population was 25,142 at the 2020 census. Located on Lake Michigan's western shore with significant commercial developments along Interstate 43, the community is a suburb in the Milwaukee metropolitan area. Despite being the third-largest city in Wisconsin by land area, approximately half of Mequon's land is undeveloped, and agriculture plays a significant role in the local economy.

When the first white settlers arrived in the 1830s, the Mequon area was inhabited by the Menominee, Potawatomi, and Sauk people. In the 1840s, German immigrants settled in the community, building farms and hydropowered mills along the Milwaukee River. Much of the community remained rural, while Thiensville developed as a market town along the local railway, providing services to the farmers. Thiensville incorporated as a village in 1910. Mequon remained rural in the early 20th century but experienced significant population growth during the suburbanization that followed World War II. The community incorporated as a city in 1957 to avoid annexation by the City of Milwaukee. The City of Mequon completely surrounds Thiensville, leading some residents to call Thiensville "Mequon's donut hole." The two municipalities have a close relationship, with a shared chamber of commerce, library, and school district.

Lutheranism has played a significant role in Mequon since the community's early years. Some of the first German settlers were Old Lutherans who founded the Freistadt community—now a neighborhood in western Mequon—in 1839 and went on to form the first Lutheran congregation in Wisconsin. In the 21st century, there are more Lutheran churches in Mequon than churches of any other single denomination. Additionally, Mequon is home to two private Lutheran post-secondary institutions: Concordia University Wisconsin and Wisconsin Lutheran Seminary. In addition to having other Christian denominations, Mequon is also the northernmost of Milwaukee's suburbs to have a sizable Jewish community.