The car was found on Friday at the end of a forest track more than 1 km (0.6 miles) from a main road in northern Sweden.
Police say the temperature in the area had recently dropped to -30oC.
The man, who was too weak to utter more than a few words, said he had been inside since 19 December. He may have survived by drinking melted snow.
Police say they have no reason to doubt his story.
The man, who has not been named, is recovering at Umea University Hospital – where staff say he is doing well considering the circumstances.
The 45-year-old was discovered by snowmobilers who initially assumed the car was a wreck until they dug their way to a window and saw movement inside, reported the Vasterbotten Courier newspaper.
The man was huddled in a sleeping bag on the back seat, said policeman Ebbe Nyberg.
“He was in a very poor state. Poor condition. He said he’d been there for a long time and had survived on a little snow.
“He said himself he hadn’t eaten anything since December,” Mr Nyberg said.
Doctors at the Umea University Hospital said they would normally expect a person to survive without food for around four weeks, said the Vasterbotten Courier.
One doctor told the newspaper that the man might have survived so long by going into a kind of hibernation.
Source: BBC News