Clark wrote a letter on DebartoloHoldings.com that revealed he first started feeling weakness in his hand in September 2015.
He said he underwent months of tests and treatment and added, "There is no test that will positively diagnose you with ALS. You have to eliminate the possibility of all other diseases and disorders and then wait to see what additional symptoms you develop."
49ers CEO Jed York released a statement regarding the news:
I was deeply saddened to learn of Dwight's recent diagnosis. As a member of our family, he will receive the full and unconditional support of the San Francisco 49ers. Many know Dwight as an iconic figure in 49ers lore, whose accomplishments on the field brought joy to fans around the world. Our organization is fortunate to know him more intimately as a wonderful man who has given so much of himself as an ambassador to the entire Bay Area. We will stand alongside Dwight and his family as they wage this battle.
Clark said his 49ers teammates have reached out to him with messages of support.
He played for the 49ers from 1979 to 1987 and was a two-time Super Bowl winner, two-time Pro Bowler and member of the 1982 First Team All-Pro.
He is best known for The Catch in the NFC Championship Game after the 1981 season. His jumping catch in the back of the end zone in the waning moments helped the 49ers beat the Dallas Cowboys, 28-27.
San Francisco went on to defeat the Cincinnati Bengals in the Super Bowl following that win.