The elderly woman found the grenade in her husband’s study and drove into St Helier before calling the police.
She was told to park at the Harbour while the Island’s bomb disposal officer was sent to the scene.
The device was later taken away to be destroyed.
In a Facebook post, the States police said: ‘This morning police, along with the bomb disposal officer, attended at the ferry terminal to recover a WW2 Mills hand grenade from a car boot. This is a British hand grenade.
‘The owner of the car had found the item in their home where it had been for decades and was taking it to the police station.
‘If you come across old firearms or military items in premises, which could possibly be dangerous, leave in situ, do not handle and call the police on 612612.’
