New York City Mayor Michael Bloomberg led the ceremony at 4 World Trade Center.
With the building open for business, the hard part will be filling the 72 stories and 2.3 million square feet of new office space in the glass building.
Officials say 40 percent of the tower, which is the second largest at the site after the still unopened One World Trade, remains unleased.
The only two tenants so far are the Port Authority and the Human Resources Administration for the City of New York.
Developers are hoping that more tenants will come forward soon.
"The World Trade Center is not only rebuilt, but we're making it a great part of New York again," said Janno Lieber, of World Trade Center Properties. "We're making all the buildings green, energy efficient and better places to work so people like to come here."
The building is also located directly across from the 9/11 memorial, so the all-glass design allows tenants to overlook the site. The building faces the Sept. 11 memorial's south reflecting pool. It's designed by prize-winning architect Fumihiko Maki.
4 World Trade might not be the tallest building, but it has the largest lobby by volume in all of New York City. The artwork inside is waterfalls and trees, some of the symbols of the memorial.