The Swiss-German child died on Tuesday at around 8:30pm a campsite in Muzzano, in the Swiss canton of Ticino, the Corriere di Ticino reported.
Medics were unable to revive the child who was pronounced dead at the scene, police said in a statement.
Police did not say why the mother had left the child in the car. According to the newspaper the mother had been staying at the campsite with her four daughters and around a dozen other people.
Ticino's public prosecutor's office has now opened an investigation into the incident.
The incident occurred as temperatures soared during a record-breaking heatwave.
The average temperatures for the first two weeks of the month have already overtaken those seen in the summer of 2003 when the average maximum for July was 32.7C.
According to the MeteoSuisse, the canton of Ticino experienced extreme heat on Tuesday afternoon and the weather service issued a level 4 warning.