Police say the unidentified woman and the dead child may have been at the recreational area in the town of La Plata all night.
Officers went to Wills Memorial Park after receiving a call at 6:55am on Friday.
Police immediately realised the toddler was dead, said the Charles County Sheriff's Office.
The child's body - which has been taken for a post-mortem - showed no signs of trauma.
The mother was taken to a hospital for a medical evaluation.
Sheriff's spokeswoman Diane Richardson told the Washington Post the child and his mother were seen at the park the day before, but it was unclear if the child was alive then.
La Plata is located about 30 miles (48km) southeast of Washington DC.
