Oshkosh (/ˈɒʃkɒʃ/ ^ⓘ) is a city in Winnebago County, Wisconsin, United States, and its county seat. It is located on the western shore of Lake Winnebago, adjacent to the much less populous Town of Oshkosh in the north. The population was 66,816 at the 2020 census, making it the ninth-most populous city in Wisconsin. The Oshkosh metropolitan statistical area, consisting solely of Winnebago County, had 171,730 residents. Oshkosh is included in the greater Fox Cities region of Wisconsin.

Oshkosh was named for Menominee Chief Oshkosh, whose name meant "claw" (cf. Ojibwe oshkanzh, "the claw").

Although the fur trade attracted the first European settlers to the area as early as 1818, it never became a major player in the fur trade. The 1820s mining boom in southwest Wisconsin along with the opening of the Erie Canal shifted commercial activity away from the Fox River Valley and Green Bay. Soon after 1830, much of the trade moved west, as there had been over-trapping in the region.

Following the publicity caused by the Black Hawk War in 1832, there was increased interest in settling Wisconsin by Whites from the East Coast, especially New York, Indiana, and Virginia, and by 1836 the cities of Milwaukee, Madison, Janesville, Beloit, and Fond du Lac were founded, with Madison the capital of a new territorial government, setting the stage for the economic and political importance of the southern part of the state. However, Oshkosh would continue to be one of Wisconsin's top five largest cities into the twentieth century.

The establishment and growth of the wood industry in the area spurred development of Oshkosh. Already designated as the county seat, Oshkosh was incorporated as a city in 1853. It had a population of nearly 2,800.

The lumber industry became well established as businessmen took advantage of navigable waterways to provide access to both markets and northern pineries. The 1859 arrival of rail transportation expanded the industry's ability to meet the demands of a rapidly growing construction market. At one time, Oshkosh was known as the "Sawdust Capital of the World" due to the number of lumber mills in the city, 11 by 1860.

During the Civil War, the 21st Regiment, Wisconsin Infantry, of the Union Wisconsin Volunteers was organized at Oshkosh, taking in many new recruits. This was one of two units organized in the state; the other was the 6th Regiment, Wisconsin Infantry, organized at Camp Randall, Madison. The 21st mustered on September 5, 1862, marching to Ohio and Louisville, Kentucky, where it participated in the fortification of Louisville later that year. It was attached to the Army of the Ohio and later to the Army of the Cumberland.

By 1870, Oshkosh had become the third-largest city in Wisconsin, with a population of more than 12,000. The community attracted a range of professional teachers, attorneys, doctors, businessmen, and others who helped it flourish. The Oshkosh Daily Northwestern newspaper (now the Oshkosh Northwestern) was founded around this time, as was the Oshkosh State Normal School (now the University of Wisconsin Oshkosh). Lumber continued as the mainstay of the city. By 1874, it had 47 sawmills and 15 shingle mills.