Customers alerted Thierry Muscat, who manages a Casino outlet in Collombey-Centre, about the infant who was spotted inside the vehicle.
Muscat rushed to the car and smashed a window to retrieve the screaming infant, who had been choking in the heat.
The incident, which occurred last Friday, underscored the risks of leaving a child unattended in a car, especially during hot summer weather.
“I was in a state of shock,” Muscat recalled in a report carried by Le Matin newspaper on Friday.
“We had immediately called the police and issued an announcement on the public address system in the shopping centre with the licence number of the car.”
Muscat decided he couldn’t wait for police to arrive and, seeing the baby in agony, decided to act.
He used a hammer to smash a window after throwing a cover on the baby through a gap in a partially opened window to protect him from the flying glass.
The baby was rescued just as police arrived.
The child was taken to a nearby dental clinic where he was re-hydrated with water using a syringe.
Muscat said the parents were shocked when they discovered what had happened.
They did not realize that leaving a child in a vehicle in such conditions could be potentially fatal, he said.
The parents are now under investigation by the Valais cantonal department of child protection, which is deciding what further steps to take in the case.
The report emerges as a heat wave is hitting Switzerland with temperatures predicted to rise to 36 degrees in the canton of Valais and several other Swiss locations by Sunday.
In a related development, Valais cantonal police reported that a two-year-old boy drowned on Thursday night after falling into his family’s swimming pool in Fully.
The child’s seven-year-old brother was also seriously injured and is being treated at Lausanne’s university hospital.
The two children fell into the pool after breaching a security gate while the mother was briefly away from home, police said.
An investigation into the accident is underway.