Fergie: 'My daughter's dyslexic'
She made the announcement this week while she was highlighting her work with an education charity during a visit to the former school of murdered schoolboy Damilola Taylor.
Princess Beatrice, 16, who is fifth in line to the throne, was diagnosed at seven, her mother said.
It is the first time her learning difficulty has been made public.
The Duchess said her daughter, who is due to sit her GCSEs this summer, has extra classes to help her with reading and writing and will continue to do so "for the foreseeable future".
"She loves history - coming from Queen Victoria and her family, she wanted to learn about history but she couldn't because she couldn't read," she said.
"She is such a kind person. She didn't get frustrated. I would have. If you don't educate, it's ignorance. I couldn't bear to condemn my daughter to no prospects.
"If you don't teach children to read and write, you are condemning them to seclusion, unemployment and crime and there's nothing more frustrating than to be unable to express yourself.
"Imagine if I didn't realise my daughter was dyslexic. She would be having such a problem.
"She now goes out in public and speaks because she has been listened to.
Doesn't every child have the right to be listened to?"
The Duchess said she thought she was a "little bit" dyslexic herself.
"I had a problem at school. I think I am fine now, but I think I did because it took me ages to read."
When asked by a pupil what her worst spelling was, she replied: "It's something like 'psychological'. I don't know whether 'psy' is spelled 'si'."
Her comments came during a tour of Oliver Goldsmith school, in Peckham, south London, where pupils with particular learning difficulties receive support from Springboard for Children, a charity of which the Duchess of York is patron.
Adrian Pritchard, chief executive of Springboard, said: "So many children have learning difficulties and the Duchess has given them a voice."