Greenfield is a city in Milwaukee County, Wisconsin, United States. The population was 35,476 at the 2000 census.
Greenfield was originally created as the Town of Kinnickinnic around 1839. On Feb. 19, 1841, the name of the town was changed to Greenfield, taking its name from the name of the Post Office in the Town of Kinnickinnic. Greenfield was the final municipality in Milwaukee County to incorporate, and was the final municipality to leave the Town form of government in 1957. This was done to prevent total annexation by the City of Milwaukee. Before 1957, Milwaukee slowly nibbled away at Greenfield's land.
Greenfield is located at 42°57′47″N 87°59′45″W / 42.96306°N 87.99583°W / 42.96306; -87.99583 (42.963034, -87.995943). The Root River flows through the western part of the city.