He was taken to hospital with a core temperature of less than 29 degrees Celsius (79 degrees Fahrenheit), which is usually fatal, and was gradually warmed over a period of hours.
Mechanics discovered the man - who remains unidentified - at Los Angeles airport late on Thursday, after the plane arrived from Papeete in Tahiti.
It was unclear how he had survived the below zero temperatures and thin oxygen at the cruising altitude of 10,700 metres (35,000 ft).
He was taken to hospital with a core temperature of less than 29 degrees Celsius (79 degrees Fahrenheit), which is usually fatal, and was gradually warmed over a period of hours.
Medical staff at UCLA Medical Centre said his survival was "little short of a miracle".
"We don't know of any other person whose body temperature dropped this low who has survived," said David Langness, a hospital spokesman at UCLA.
Earlier cases of people trying to stow away in wheel wells have usually ended in death from hyporthermia or crushing in the hydraulic works of the landing gear.
Doctors said the man was in his late 20s, 1.80 metres (6ft) tall, weighing 80kg (180 lbs), but his identity is still unknown.
All communication by the man has been by written notes, in French and English.
Doctors are trying to determine whether oxygen deprivation has damaged the man's brain.
Obscure
Officials from the US Immigration and Naturalization Service have yet to interview him, but said that the motives for his action were obscure.
"It is fairly easy for Tahitians to get non-immigrant visas, to come to the US as visitors. We don't see a high incidence of attempts at illegal immigration from Tahiti," said INS spokeswoman Virginia Kice.
She added that the airline was responsible for the care and custody of anyone on board the plane, and ensuring he was properly inspected at the port of entry.
Air France said they did not know how the man got into the wheel well in the first place.
"This is very rare. Security at airports has gotten very good and it's very difficult for an unauthorised person to get near a plane or to stow away," said Air France spokesman Dean Breest.
Airport officials in Papeete said the man had evaded controls by wearing a stolen uniform of airline ground staff, without specifying the airline.