Authorities said the one-to three-month-old couldn't have gotten over the ledge on his own, which makes the circumstances suspicious.
He said a witness called 911 after seeing the baby drop from the second or fourth floor of the Children's Hospital Orange County garage.
The boy suffered critical injuries and was rushed to a trauma center at UC Irvine Medical Center for treatment.
Police had no information on the baby's identity or that of his parents. It was unclear whether the baby had been treated at the children's hospital.
Detectives were reviewing surveillance video to determine what happened and locked down the garage to search for evidence.
Anyone with information is urged to call the Orange Police Department at (714) 744-7444.
The Associated Press contributed to this report.