A resident discovered the explosive last Thursday afternoon while fetching newspapers from a mailbox in the village of Sundby in central Ostrobothnia.
Detained by police on Friday, a local middle-aged man confessed that he placed the mortar round in the box.
The perpetrator was unable to explain his motive, but heavy drinking is suspected to have played a part in the incident. He had acquired the mortar round from a close relative who had brought it home as a souvenir during WWII.
If the device had exploded, it could have caused serious injuries or possibly deaths.