Investigators say they're trying to identify and find the mother for medical reasons. They say she may be in distress and losing a lot of blood.
Los Angeles County Fire personnel arrived at the park near the 60 Freeway and the 1200 block of Lerma Road, where they found the body.
A man doing community service as a grounds maintenance worker called 911 around 8 a.m. after making the discovery next to a trash can.
"This is a 6- to 7-month gestational period baby found with an umbilical cord attached to the placenta," sheriff's Lt. Hugo Reynaga told reporters at the scene. "That's all we have at this point. There's no cameras out here, there's no video cameras in the park that would give us any more information."
Details were not immediately available on the infant's identity or cause of death.
"Investigators believed the infant had been abandoned for approximately 3 hours before he was discovered," the sheriff's department said in a news release. "There was no traumatic injury observed on the infant."
Detectives say it's unclear whether the mother will face any criminal charges.
"This shouldn't have happened. Unless the female was in some kind of distress and this happened unknowingly to her," Reynaga said.
Reynaga reminded people about California has a Safely Surrendered Baby Law, which allows parents to safely surrender their newborns at a hospital, fire station or other designated site within 72 hours of birth.