Aiden Barrios was sitting in a car when it was stolen after his mum Elizabeth left it running while she was dropping off another child at a nursery.
When the mum walked out of the nursery, the horrified mother saw her car being driven away with little Aiden inside.
Frantic Elizabeth then realised she'd left her phone inside the vehicle and alerted police who called her number to try to talk to the thief and persuade them to release the toddler unharmed.
But to their surprise – and Elizabeth's huge relief - Aiden answered the phone and told his mum that he was now alone in the parked car after the thief abandoned the vehicle and fled.
Police instructed distressed Aiden to start honking the horn to alert nearby officers of the car's location.
The youngster - who was unharmed by his ordeal - was reunited with his mum around 20 minutes after the car was stolen.
Detectives in Ogden, Utah, US are still searching for the suspect.
Speaking to local TV news, Elizabeth recalled her reactions when she saw the thief taking off with her car."My baby - what are they going to do to him?" she said.
Police hailed Aiden's bravery, and described him as a 'very smart kid' who did a 'great job'.
He was awarded an honorary junior police officer award for his "heroic acts".
Police have advised the public not to leave a running vehicle unattended.
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