The mortar was found in Coed y Wenallt at Nant Gwynant on Tuesday and handed in to officers.
Bomb disposal experts carried out a controlled explosion at Brewery Fields in the city on Wednesday morning.
A North Wales Police spokesman said officers cordoned off the area to ensure there was no danger to the public.
An unexploded World War Two bomb was taken to Bangor police station by a member of the public, police have said.