They said the father of the child was supposed to drop the baby off at a daycare center after taking his older children to school but apparently forgot she was in the car and went about his day.
The dead baby was discovered Tuesday evening when the child's brother climbed into the back seat of the car after school, The Miami Herald reported.
The baby was pronounced dead at Miami Children's Hospital.
The father told police it was not part of his routine to care for the baby.
No charges were immediately filed.