She was born Elizabeth Bowes-Lyon, on 4 August 1900.
The palace say she died peacefully in her sleep at 3.15pm on Saturday at the Royal Lodge, her Windsor home.
Her daughter, Queen Elizabeth II, was by her side.
The Queen Mother had a varied and long life and was one of the best-loved Royals.
But she had suffered from ill health for some time. She had been trying to recover from a chest infection since Christmas.
In the year 2000, thousands of people came to London to celebrate her 100th birthday.
But in February 2002, she was deeply saddened by the death of her daughter Princess Margaret at the age of 71.
The Queen Mother will be greatly missed by her family and the public.