It happened in Mesa, near Mesa Drive and Brown Road Wednesday afternoon.
Firefighters took the 2-year-old girl to an area hospital, where he was reportedly conscious and breathing.
The mother of the child said she simply forgot her child was in the car while she went shopping.
She had three other kids with her, but left her 2-year-old in the car.
Officers say they believe the child was in the car for 15 minutes.
Mesa Police questioned the mother but let her go to be with her daughter at the hospital, it's unclear if she will face charges.