Greenfield is a city in Milwaukee County, Wisconsin, United States. The population was 37,803 as of the 2020 census. Greenfield is one of many bedroom communities in the Milwaukee metropolitan area.

Milwaukee County is located in the U.S. state of Wisconsin. At the 2020 census, the population was 939,489, down from 947,735 in 2010. It is both the most populous and most densely populated county in Wisconsin, and the 45th most populous county nationwide; Milwaukee, its eponymous county seat, is also the most populous city in the state. The county was created in 1834 as part of Michigan Territory and organized the following year.