Nashville police say the adoptive father forgot to drop the 1-year-old girl off at daycare Wednesday.
The girl's mother later found the baby in a car seat inside the truck, which was parked outside their home. The girl was taken to a hospital where she was pronounced dead.
Tennessee is having a heat wave. The high temperature was 88 degrees.
It's not clear how long the girl was in the truck. A police investigation is ongoing.