CHICAGO — The Sears Tower, one of the world's iconic skyscrapers and the tallest building in the United States, was renamed the Willis Tower today in a ceremony, marking a new chapter in the history of the giant edifice that has dominated the Chicago skyline for nearly four decades.

Mayor Richard Daley and Joseph Plumeri, who heads Willis Group Holdings, the London-based insurance broker that secured the naming rights as part of its agreement to lease 140,000 square feet of space in the tower, unveiled a sign with the new name in the tower's lobby.

"Willis is investing in Chicago. Willis is investing in Chicago. That's positive," Plumeri said during today's ceremony.

Plumeri said the company plans to bring hundreds of jobs to the city and help in the community.

"You will find over time that Willis is not going to just have its name on the building," Plumeri said. "I promise you that."

The 110-story skyscraper might officially have a new name for the first time since its 1973 opening.

The tower's original tenant, Sears Roebuck and Co., moved out in 1992 but its name remained. A real estate investment group, American Landmark Properties of Skokie, now owns the 1,450-foot-tall building.