The piece, which was recovered by chance in December in the northern Italian city of Piacenza, was stolen 23 years ago.
Why the painting was left in the wall at the Ricci Oddi Gallery of Modern Art is still a mystery.
It was discovered by gardeners while they were clearing ivy - and experts have now confirmed its authenticity.
On Friday, it was declared authentic by experts following technical testing of the work, municipal mayor Patrizia Barbier said in a statement.
Portrait of a Lady was officially listed as missing on February 22, 1997 but might have been snatched from a gallery wall a few days earlier, during the exhibit preparation work. Selleri said the radiation analysis revealed that while painting the later portrait, Klimt didn't redo much of the face, but used whitish pigment from the earlier version for the skin.
During the announcement, it was displayed on an easel and flanked by two police officers.
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"The correspondence between the images allowed us to determine that it's definitely the original painting", art expert Guido Cauzzi said.
Experts said the painting was in remarkably good condition.
The museum's president Massimo Ferrari had told AFP, after the lost painting was first discovered in the wall in December, that there were "positive signs" that this was the genuine Klimt.
ABC News reported last month that the painting was believed to have been stolen in 1997 while it was being moved into another building. Chicca said police were studying some organic traces found on the canvas in hopes it might lead investigators to the culprits.
As for why and when the painting ended up stashed behind a wall, journalist Anne-Marie O'Connor, the author of a book about the dramatic fortunes of Klimt's "Portrait of Adele Bloch-Bauer", has an educated theory. That was not the case as the skylight was too small for the painting to fit through. Klimt had painted over the portrait when the girl died suddenly, to forget the pain of her death.
It has been reported that the painting - of a woman with dark hair and red cheeks - could fetch around $A100m if it was sold.