Officers said they found the boy lethargic and sweating when they pulled him out of the car.
"Every summer you get reports that parents have left babies in vehicles on hot days like this, with the results ending tragically," said LAPD Sgt. Paul Hendry.
He was taken to a local hospital and is expected to be fine.
The child's mother was inside Babies "R" Us shopping for a gift. Police estimate that she was gone for about ten minutes.
Police arrested the mother for child endangerment.