Police found an emaciated four-year-old boy in a flat with the body of his mother, who appeared to have been dead for about five days.
The boy tried to get food from the refrigerator but could not open the door, the police director in Union, northern New Jersey, said.
He said the boy had rubbed lotion on his mother in an apparent attempt to help her, leaving behind his handprints.
The discovery was made after a maintenance worker reported a bad smell coming from the flat. The door was ajar but locked with a chain.
The woman was found in a bedroom. Police do not suspect foul play.
The boy was taken to hospital for treatment.