Alan Nierob, the Los Angeles-based publicist for Richardson's husband Liam Neeson, confirmed her death in a written statement.
"Liam Neeson, his sons, and the entire family are shocked and devastated by the tragic death of their beloved Natasha," the statement said.
"They are profoundly grateful for the support, love and prayers of everyone, and ask for privacy during this very difficult time."
The statement did not give details on the cause of death for Richardson, who suffered a head injury when she fell on a beginner's trail during a private ski lesson at the luxury Mont Tremblant ski resort in Quebec. She was hospitalized Tuesday in Montreal and later flown to a hospital in New York City.
Family members had been seen coming and going from the New York hospital where Richardson was reportedly taken.
Vanessa Redgrave, Richardson's mother, arrived in a car with darkened windows and was taken through a garage when she arrived at the Lenox Hill Hospital on Manhattan's Upper East Side at around 5 p.m. Wednesday. An hour earlier, Richardson's sister, Joely, arrived alone and was swarmed by the media as she entered through the back of the hospital.
Richardson, 45, part of the Redgrave dynasty of British actors and the wife of Liam Neeson, was flown from Montreal to New York on Tuesday after the accident, a person close to the family, who asked not to be identified because of the sensitivity of the matter, told The Associated Press.
A reporter from the Toronto Star reported seeing a distraught Neeson crouched inside the back of an ambulance Tuesday at Montreal's Sacre-Coeur hospital as Richardson, wrapped in blankets and with tubes covering her face, was loaded inside. Neeson had immediately left the Toronto set of his upcoming movie, Chloe, to be by her side in Montreal, a publicist for the film said.
A statement from the Mont Tremblant resort said Richardson fell on a beginners trail and later reported not feeling well.
"She did not show any visible sign of injury but the ski patrol followed strict procedures and brought her back to the bottom of the slope and insisted she should see a doctor," said the statement from the resort, about 80 miles northwest of Montreal.
The ski resort said the instructor and a member of the ski patrol accompanied Richardson to her hotel, where they again recommended she be seen by a doctor. Mont Tremblant spokeswoman Catherine Lacasse said Richardson said she seemed fine at first.
"An hour later she said she didn't feel well. She had a headache, so we sent her to the hospital," Lacasse said.
"There were no signs of impact and no blood, nothing."
Richardson is the elder daughter of Oscar-winning Redgrave and the late director Tony Richardson.
Richardson's films include Gothic, A Month in the Country, Nell (in which she appeared with her future husband), The Parent Trap and Maid in Manhattan.
Trained at London's Central School of Speech and Drama, Richardson has had extensive stage experience in the West End and Broadway. She won a Tony in 1998 for playing Sally Bowles in a revival of Cabaret.
Her maternal grandparents were the actors Michael Redgrave and Rachel Kempson, and her uncle Corin and aunt Lynn Redgrave are both actors. Joely Richardson is also an actress, best known for starring in the TV series Nip/Tuck.
In January, Richardson and her mother played the roles of mother and daughter in a one-night benefit concert version of A Little Night Music, the Stephen Sondheim-Hugh Wheeler musical, at Studio 54 in New York.
She married Neeson in 1994; their sons are 13 and 12.
