The mother is employed as a waitress at a Hooters restaurant near Bell Road and Interstate 17, Phoenix police said.
She arrived at the restaurant at 10:00 a.m and worked until nearly 5 p.m. police said.
The baby was in a car seat in the backseat of a Honda Civic. The car has dark tinted windows.
Officials think that that made the boy invisible to people going in and out of the restaurant and a carwash next door. The car was parked in the front of the lot on Bell Road and about 20 feet from the Hooters patio seating area.
The mother walked to her parked car, unlocked the driver's door, sat in the vehicle and immediately realized that she had forgotten that her 17 month old child was in the back seat of her car, police said.
The mother immediately summoned help, but efforts by witnesses, police officers and fire fighters were unsuccessful and the child was pronounced dead, Phoenix police said.
"Temperatures hit nearly 90 degrees Tuesday, but the temperature inside the car reached more than 100 degrees," said Sgt. Joel Tranter, a Phoenix police spokesman.
Preliminary information released by Phoenix police says the mother was to drop the child off at child care on her way to work Tuesday morning, but for unknown reasons failed to do so.
Phoenix police investigators are conducting a death investigation at this time, police said.