The Illinois Steel Company was an American steel producer with five plants in Illinois and Wisconsin. Founded through a consolidation in 1889, Illinois Steel grew to become the largest steel producer in the United States. In 1898, several other steel and transportation companies were merged into it to form the Federal Steel Company, itself merged into U.S. Steel in 1901.

United States Steel Corporation, more commonly known as U.S. Steel, is an American integrated steel producer headquartered in Pittsburgh, Pennsylvania, with production operations in the United States and Central Europe. It was the 8th largest steel producer in the world in 2008. By 2018, the company was the world's 38th-largest steel producer and the second-largest in the US, trailing only Nucor Corporation.

Though renamed USX Corporation in 1986, the company was renamed United States Steel in 2001 after spinning off its energy business, including Marathon Oil, and other assets from its core steel concern.