"I have ALS, also known as Lou Gehrig's disease," the 60-year-old Clark wrote. "Those words are still very hard for me to say."
Clark wrote that he was diagnosed with ALS (amyotrophic lateral sclerosis) in late 2015 after visits to six neurologists and three ALS specialists.
"The one piece of good news is that the disease seems to be progressing more slowly than in some patients," Clark wrote. "While I'm still trying to wrap my head around the challenge I will face with this disease over the coming years, the only thing I know is that I'm going to fight like hell and live every day to the fullest."
Clark thought something might be wrong when he noticed a weakness in his left hand in September 2015. At first he was told it was a B-12 vitamin deficiency.
"I was mildly paying attention to it because since my playing days, I've constantly had pain in my neck," Clark wrote. "I was thinking it was related to some kind of nerve damage because it would just come and go."
Clark, a two-time Pro Bowl selection, played nine seasons with the 49ers, winning two Super Bowls. "The Catch" came on a pass from Joe Montana, and it sent the 49ers to their first Super Bowl, which they won.
His 6,750 receiving yards rank third in franchise history behind only Jerry Rice and Terrell Owens. The 49ers retired Clark's No. 87.
Clark later served as a team executive for the 49ers, and he was the general manager and director of football operations for the Cleveland Browns from 1999 to 2002.
"I've been asked if playing football caused this. I don't know for sure. But I certainly suspect it did," he wrote. "And I encourage the NFLPA and the NFL to continue working together in their efforts to make the game of football safer, especially as it relates to head trauma.
"I can't run, play golf or walk any distances. Picking up anything over 30 pounds is a chore," he wrote. "In addition to losing strength in my left hand -- which makes opening a pack of sugar or buttoning my shirt impossible -- I have now experienced weakness in my right hand, abs, lower back and right leg."
"What I do know is I have a huge battle in front of me and I'm grateful for the strength and unconditional love from my wife Kelly. She has been my rock," Clark wrote. "She keeps thinking positive and convinces me each day that we can beat this, as does my daughter Casey and my son Mac. My brother Jeff, his wife Debra and their family also have been unwavering with their love and support."
"I've got to devote all my energy preparing for this battle and I would hope you can respect my family's privacy as I begin this challenge," Clark wrote. "My ultimate hope is that eventually I can assist in finding a cure for ALS, which disrupts the lives of so many and their loved ones."