It happened around 10 p.m. Wednesday at Cottman Avenue and Horrocks Street in Northeast Philadelphia.
Police say a male suspect stole a 2013 blue Volkswagon Passat with the 9-month-old boy asleep in the back seat.
According to Philadelphia Police Chief Inspector Scott Small, the baby's mother left the vehicle running while she was going inside a takeout restaurant. As she exited the vehicle, police say the suspect jumped in and took off.
Around 10:30 p.m., police say the child and the vehicle were found near Tabor Road and Adams Avenue in the city's Crescentville section.
"We had all of our police units in the area looking for this vehicle, along with our helicopter, our aviation unit, and they did a great job by finding the vehicle about four miles from this location," said Small.
The 9-month-old was not injured.
Police are still searching for the suspect.
Anyone with any information is asked to call police at 215-686-TIPS.