The woman landed in a sitting position on the roof of a taxi whose driver got out just before the impact deeply dented his roof and shattered the windshield.
The woman, a 30-year-old Argentine, has injuries throughout her body and is being treated in the emergency room of the Hotel Argerich, said Alberto Crescenti, director of Argentina's Emergency Medical System.
The taxi driver, who gave his name as Miguel, reportedly said he saw a policeman looking up and that prompted him to get out just before the driver's side of the car was smashed by the woman's body.
Another taxi driver, Juan Carlos Candame, told Associated Press Television News that he saw the woman climb over a barrier and jump into the void.
The woman fell from the top of the Hotel Crown Plaza Panamericano, where a restaurant overlooks the landmark Obelisk in downtown Buenos Aires.