Both men are believed to have clung on to a British Airways plane as it flew more than 8,000 miles from Johannesburg in South Africa to Heathrow.
The victim fell on to a shop roof in Richmond, which is below the Heathrow flight path, on Thursday morning.
A Scotland Yard spokeswoman said: “The death is currently being treated as unexplained. A post-mortem examination will be held in due course and inquiries are ongoing to establish the male’s identity. No arrests have been made.
“In regards to the male who survived, police were alerted at 8.28am on 18 June to reports of a suspected stowaway on a flight from Johannesburg to Heathrow.”
The man who survived is aged between 25 and 30 and in a serious condition in hospital.
A British Airways spokeswoman said: “We are working with the Metropolitan Police and the authorities in Johannesburg to establish the facts surrounding this very rare case.”
There have been other cases of stowaways plunging to their deaths from the undercarriages of Heathrow-bound flights, being found dead at the airport, or even surviving.
Jose Matada was found dead on the pavement in East Sheen, near Richmond and also under the Heathrow flight path. He is believed to have died after stowing away inside the wheel recess of a British Airways plane travelling from Angola.
Matada, who died on his 26th birthday, was either dead or near death due to hypothermia and lack of oxygen when he fell from the undercarriage of a Boeing 777 as it was descending.