The man was injured during an attempted break-in at a branch of McDonalds in the town of Abtwil on Sunday morning at around 6:15 am, 20 Minutes reported.
According to reports, the intruder, who was wearing a hood, fired several shots into the building from the outside. He did not manage to enter the building.
The intruder then took flight empty handed, Saint-Gall police said.
The victim, aged 22, was taken to hospital.
Further details could not be disclosed as cantonal police continue their investigation.