Bank One Corporation, based in Chicago, Illinois, was the sixth-largest bank in the United States. It traded on the New York Stock Exchange under the stock symbol ONE. The company was acquired by JPMorgan Chase & Co. on July 1, 2004.

The bank traces its roots to First Bancgroup of Ohio, founded as a holding company for City National Bank of Columbus, Ohio and several other banks in that state, all of which were renamed "Bank One" when the holding company was renamed Banc One Corporation. With the beginning of interstate banking they spread into other states, always renaming acquired banks "Bank One", though for a long time they resisted combining them into one bank. In 1998, Banc One Corporation merged with Chicago-based First Chicago NBD Corporation to form Bank One Corporation, and headquarters moved from Columbus to Chicago. Adverse financial results led to the departure of CEO John B. McCoy, whose father and grandfather had headed Banc One and predecessors. Jamie Dimon, a former key executive of Citigroup, was brought in to head the company.