The spire's installation was completed Friday morning, after pieces of it had been transported to the roof of the building last week. Construction workers below applauded the milestone.
The 1,776 feet - or 541 meters - is symbolic of the year 1776, when the US declared its independence.
"It's a pretty awesome feeling," said project manager Juan Estevez from a temporary platform on the roof of the tower where he and other workers watched the milestone.
"It's a culmination of a tremendous amount of team work ... rebuilding the New York City skyline once again."
The building is rising at the northwest corner of the site where the twin towers were destroyed in the Sept. 11, 2001, terror attacks. The area is well on its way to reconstruction with the 72-story Four World Trade Center and other buildings.
The 408-foot (124.36-meter) spire, weighing 758 tons, will serve as a world-class broadcast antenna. An LED-powered light emanating from it will be seen from miles away and a beacon will be at the top to ward off aircraft.
The addition of the spire, and its raising of the building's height to 1,776 feet, would make One World Trade Center the tallest structure in the US and third-tallest in the world, although building experts dispute whether the spire is actually an antenna - a crucial distinction in measuring the building's height.
If it didn't have the spire, One World Trade Center would actually be shorter than the Willis Tower in Chicago, which stands at 1,451 feet (442.26 meters) and currently has the title of tallest building in the US, not including its own antennas.
The Council on Tall Buildings and Urban Habitat, a Chicago-based organization considered an authority on such records, says an antenna is something simply added to the top of a tower that can be removed. By contrast, a spire is something that is part of the building's architectural design.
The tower is slated to open for business in 2014.
Tenants include the magazine publisher Conde Nast, the government's General Services Administration and Vantone Holdings China Center, which will provide business space for international companies.