The police bomb squad was called to 2 World Financial Center in lower Manhattan at midday when a security guard reported a package that seemed suspicious. Brookfield Properties, which runs the property, ordered an evacuation as a precaution.
Employees in business suits filed calmly back into the building after the New York Police Department declared the situation all clear about 90 minutes later.
Police said there was no threat, and no criminal investigation is taking place.
The bomb squad determined the package was a fake explosive that looked like a 1940s-style pineapple grenade. It was mounted on a plaque that said "Complaint department: Pull pin."
It was addressed to someone at one of the financial institutions housed there.
Several employees said an announcement came over the building loudspeakers telling them to evacuate as quickly as possible. Octavio Diaz was wearing a neon yellow backpack as he helped lead his co-workers out of the building to a nearby volleyball court, where they waited until the all-clear.
"Stuff like this happens, so you've got to take it seriously," he said. "We're ready to go."
The building is near the World Trade Center site, where the Sept. 11 attacks killed nearly 3,000 people. A 1993 bombing there killed six people and injured more than 1,000.
The building evacuated Thursday houses major financial institutions, including Merrill Lynch, Nomura Securities, Deloitte, Commerzbank and OppenheimerFunds Inc.