Simmons confirmed the news as he prepares to embark on a charity walk of close to 300km from his home town of Goolagong to Penrith to raise money for Dementia Australia and raise awareness of the disorder.
The former Penrith captain said he was unsure if the disease was linked to multiple concussions he had suffered throughout his career.
The 61-year-old is the latest rugby league player to confirm he has a brain injury, with Steve Mortimer the most recent high-profile case last year.
Simmons said he became aware of the issue when he forgot his wife had purchased a bottle of wine just minutes after they had visited a shop.
It prompted him to visit a neurologist last June for scans.
“The doctor said, ‘You told me you’d had a few concussions over the years; there are a couple of white marks which may have been from the concussions’,” Simmons said in an interview with Neil Cadigan on the Panthers’ website.
“And I’m thinking, ‘That’s good; just a couple of marks from concussions’.
“Then he pointed to another part of the brain and said, ‘Around here, that’s all Alzheimer’s’.
“It was like someone had knocked me off my feet.
“The rest of the appointment is just a blur; I was so shocked, even though in the back of my mind I was worried something might show up.”
Simmons said his condition had not deteriorated significantly since his diagnosis, and he is still at Penrith in an ambassador position.
He is still able to exercise, leaving him in no doubt he can complete the 10-day walk and arrive at Bluebet Stadium for the Panthers’ clash with North Queensland on May 27.
Fan-favourite Simmons played 238 games for Penrith, vowing to have a beer with every fan after a try-scoring double in his farewell grand final as the club secured their maiden title in 1991.
“You have all the stats about people who suffer from dementia or die from it but you’ve got to multiply that by 10 or 20 people who suffer as well alongside them,” Simmons said.
“I want to help raise money for research as a mark of respect for the medical people who have worked so hard find a cure or a successful treatment and all those people who suffer watching a loved one go down the path of losing who they once were.”