Upper Merion Police and firefighters were called to The Court at King of Prussia around 1:45 p.m. Wednesday after a mall worker discovered the device inside a planter or flower pot by the Eddie Bauer store.
Police say about one quarter of The Court was evacuated as a precaution.
Investigators soon discovered the replica grenade was not an explosive device and posed no danger.
However, police are still searching the rest of The Court to see if there are any other devices. They also want to know who left the original device behind.
Police say the situation is now under control and there have been no injuries.
Customers have been allowed back inside the mall.