The body was discovered after a worker at the lot reported a strange odor coming from the red convertible Pontiac and called police, The Washington Post reported Friday.
D.C. Police Chief Cathy L. Lanier declined to comment on the car's registration.
The remains were taken to the medical examiner's office for an autopsy.
Lanier said the car was towed to the impound lot because it was on a city street in violation of rush-hour rules.
At the time, there was no reason to suspect anything other than a parking infraction, Lanier said.
"Obviously we don't know much right now," Lanier said. "We're investigating this as an undetermined death at this point."