St. Croix County is a county in the U.S. state of Wisconsin. As of the 2010 census, the population was 84,345. Its county seat is Hudson. The county was created in 1840 and organized in 1849.St. Croix County is part of the Minneapolis-St. Paul-Bloomington, MN-WI Metropolitan Statistical Area. Between 2000 and 2010, it was the fastest growing county in Wisconsin.