The child's parents and other family members were questioned Saturday but no arrests were made at the time, said police in the San Diego suburb of El Cajon.
Temperatures in El Cajon reached 80 degrees Saturday, KNSD-TV, San Diego, said.
Police said the child was unresponsive when they got him out of the car, which was parked at an apartment complex. He was rushed to a hospital but died a short time later.
Larry Singleton, a resident of the complex, told KNSD he called police when a group of neighborhood children noticed the baby in the car. He said the vehicle belonged to a family that had recently moved in, and he estimated the car could have been parked for several hours.