Police said relatives identified the woman as Natascha Kampusch, who vanished in 1998 at age 10 while walking to school.
The 44-year old man suspected of having kidnapped and imprisoned the girl has committed suicide.
Police say the communications technician threw himself in front of a train in Vienna.
The now 18-year old woman showed up yesterday in a garden close to the house where she'd been kept and identified herself to a neighbour.
Austrian news agency APA quoted the investigators as saying there were no indications the woman had suffered sexual abuse.
The woman, very pale but in apparently good physical health, told police she had escaped earlier in the day from a house in a village near Vienna where she had been largely confined in an secured garage since her abduction.
She said her captor would occasionally allow her to take walks with him through his neighbourhood and that she had access to radio, television, newspapers and books, and the garage was equipped with a bed and wardrobe, according to police and local media.
Woman may suffer 'Stockholm Syndrome'
BKA investigator Erich Zwettler, when asked why the woman had not fled while on any of her outings, said she seemed to have had "Stockholm Syndrome", a psychological condition in which long-held captives begin to identify with their captors.
"She is white-pale, looking as if she had been out of the light of day for a long time, but she articulated well and could read and write," APA quoted a police investigator as saying.
Neighbourhood witnesses said they saw a car speeding away after the woman approached residents close to the house where she said she had been held, and police later found the vehicle abandoned in an underground garage.
Ms Kampusch's disappearance was widely publicised because it occurred at a time when Europe was unnerved by a notorious case of child abduction and murder in Belgium. A nationwide search, including dragging of riverbeds, found no trace of her.