Joliet is a city in Will and Kendall counties in the US state of Illinois, located 40 miles (64 km) southwest of Chicago. It is the county seat of Will County. As of the 2010 census, the city was the fourth-most populated in Illinois, with a population of 147,433. It continues to be Illinois' fastest growing city and one of the fastest growing in the southwest Chicago metropolitan area. When Joliet was first planned in the early 1830s, it was still in Cook County. In 1836, it became the county seat of the new Will County.