The world's largest airline already has extensive operations inside the skyscraper. Willis Tower is home to United's Network Operations Center. Now, United will move its corporate headquarters from another downtown building to the Willis Tower.
The air carrier will occupy 16 floors -- approximately 25 percent-- of the building and, eventually, have more than 4,000 employees working in Willis Tower. United will lease a total of 830,000 square feet.
In announcing the deal Monday, Chicago Mayor Rahm Emanuel said United's decision to extend its commitment to the city's downtown is "great news for all Chicagoans."
United executive Brett Hart praised Willis Tower for providing "a dynamic work environment" for the airline's employees.
(The Associated Press contributed to this report.)