The 41-year-old said he had received the diagnosis in December last year, with doctors telling him he was one of the worst cases they had seen. He was also diagnosed with probable chronic traumatic encephalopathy.
Jones won 75 caps for Wales and captained the side to a Six Nations Grand Slam in 2008, making his last appearance for them in 2014. He was also part of the British and Irish Lions squad that toured New Zealand in 2005.
"I feel like my world is falling apart. And I am really scared," Jones said in the Sunday Times. "I lived 15 years of my life like a superhero and I'm not. I don't know what the future holds.
In 2020, a group of former rugby players filed a class-action legal case against World Rugby and other governing bodies, alleging that their failure to protect them led to early onset of dementia.
The rugby governing body has announced guidelines limiting full contact training to 15 minutes per week and launched a brain health education campaign for players.
Photo: PHOTOSPORT Caption; Ryan Jones in 2008