City police in Winterthur, in the canton of Zurich, were alerted at around 8pm on Sunday to the presence of a baby boy in a car stationed in an underground carpark.
The locked car's windows were closed but the sunroof partially open, police said in a statement.
Police released the unharmed baby from the car around an hour after he was left.
A complaint has been filed against the parents, a 34-year-old man and a 24-year-old woman, who said they left their baby son in the car because he had finally fallen asleep, according to police.
The incident comes just a month after the tragic case of a young girl left by her mother in a car in a campsite near Lugano, in the canton of Ticino, during the July heatwave.
The five-year-old Swiss-German girl died from overheating, an autopsy confirmed.