Workers put up the flag Saturday on the south side of 1 World Trade Center.
The 1,776-foot tower is under construction.
The 60-by-90-foot flag typically flies at the George Washington Bridge on major holidays.
The Port Authority of New York and New Jersey says it's the world's largest free-flying American flag.
Another flag was placed at 1 World Trade Center on Friday.
That flag was to be moved to one of the highest points on the building.
It is already 1,000 feet tall and the tallest building in lower Manhattan.
As for the bridge, the Port Authority has a similar flag to fly there on Sunday.