The police bomb squad was called to 2 World Financial Center in Lower Manhattan at midday Thursday when a security guard reported a package that seemed suspicious.
The NYPD declared the situation all clear about 90 minutes later.
Brookfield Properties, which runs the property, ordered an evacuation as a precaution.
The building is near the World Trade Center site. A 1993 bombing there killed six people and injured more than 1,000. The Sept. 11, 2001, attacks killed nearly 3,000 people.
The World War era grenade was mounted on a plaque, with a "Pull in for Service" inscription. It was found in the mail room.
Employees were allowed back in the building around 12:15 Thursday.
Police said there was no threat, and no criminal investigation is taking place.
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.
(The Associated Press contributed to this report)
Get Eyewitness News Delivered