Willis is a global insurance broker that is the biggest tenant to move into the Sears Tower since 9/11.
Getting the naming rights is part of the deal that Willis struck with the building's owner.
It is 110 stories and nearly four million square feet of the most recognizable office space in the world. The Sears Tower is the tallest building in the western hemisphere and its sky deck draws hundreds of thousands of visitors a year.
The tower itself of is going nowhere. But it will soon be known as the Willis Tower after it's newest tenant, Willis Group Holdings, an insurance broker based in London. Five hundred Willis employees will take over several floors of the building but still will occupy only about four percent of the building.
"We found the great opportunity, a great deal in the Sears Tower, and the deal we couldn't refuse, as they say," said Joe Plumieri, Willis Group Holdings.
It's been years since Sears left the building for new headquarters in the suburbs. And city leaders say the move is good for Chicago.
"They bought the building and they have a right to it. They're putting money into it. They have announced great plans for it," said Mayor Daley.
"It also attracts another great global company to Chicago which is what our primary goal is," said Jerry Roper, Chicagoland Chamber of Commerce.
When it was built in 1973 the tower was known for its architectural marvel, what's known as bundle tube construction. It's actually nine separate towers wrapped together. And whatever name you choose to call it historians say the building will remain significant.
"There's nothing to be lost in the name change here. The tower is still significant, whether it's the Sears or the Willis Tower," said John Russick, Chicago History Museum.