The actor and singer, 66, told People he had been in “denial” but he has now accepted his condition.
Cassidy, who starred as Keith Partridge in 1970s TV series The Partridge Family, said “part of me always knew this was coming”, as the disease ran in his family.
He said as his mother and grandfather both suffered “I feared I would end up that way”.
Cassidy, who has had a tumultuous few years, recently sparked speculation he was struggling to remember lyrics at one of his gigs.
The star, who enjoyed hits in the 1970s like How Can I Be Sure, said he had decided to stop touring, adding: “I want to focus on what I am... I want to love. I want to enjoy life.”
In 2014 a judge sentenced Cassidy, who divorced his third wife last year and was recently declared bankrupt, to three months in rehab and five years’ probation for drink-driving.