The police bomb squad was called to 2 World Financial Center around midday when a security guard reported seeing a suspicious package. Law officials said a novelty hand grenade was found in the mail room.
ABC News reports the World War-era grenade was mounted on a plaque with an inscription that read, "Pull pin for service."
The NYPD declared the situation cleared in about 90 minutes.
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 Associated Press and ABC News contributed to this report.