Clark was on the receiving end of "The Catch," one of the most famous plays in NFL history. In 1982 it helped the 49ers get to the team's first Super Bowl. Clark went on to become a pro bowler -- and now has his 49ers jersey retired.
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He said Sunday in an open letter that he started experiencing the symptoms of ALS in his left hand back in 2015.
Current 49ers CEO Jed York released a statement that said he was "deeply saddened" upon learning of Clark's diagnosis and that he would receive full, unconditional support from the team and staff.
"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," York said.
We spoke with a professor of neurology at UCSF. She told us ALS is a rare disorder that affects the function of nerves and muscles. It affects one in 100,000 people and is not genetic.
Doctors still don't know exactly what causes it.
"Something triggers the nerve to die and we don't know what that trigger is," said Catherine Lomen-Hoerth with UCSF. "If we did, we could treat these diseases much better than we do."
Drugs can slow down its progression, but there is no cure. She says most people live three to five years after the first symptom of the disease.
RELATED: The ALS Association Golden West Chapter
ABC7 sportscaster and former 49er Mike Shumann was right there hugging him after that catch. They are still good friends today.
"As I look at that video, Matt, 35 years ago Dwight and I were 20 somethings headed to our first Super Bowl and winning. The first of five, and to this day, it's a bond you share for life. I've known for a while Dwight was dealing with ALS. But to see him put out this letter was just devastating, as it seemed to become real."
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Shu went on to say that Clark is not the only former player who's dealing with hardships that are possibly related to football, "He's not sure, but he thinks it probably does. You know, ALS, dementia, Parkinson's and Alzheimer's are all things we're all dealing with. We're all dealing with neurologists now, and basically going to find out I don't want to say our death sentence but we're all finding out what we're doing to be dealing with down the line. And I said it earlier, in our 30s we talked about the glory days, 40s all our injuries, in the 50s we can't remember anything, and now in our 60s we seem to be dealing with a lot of this. So a lot of players are going through the same challenges Dwight is."
VIDEO: ABC7's Mike Shumann reacts to Dwight Clark's ALS announcement:
ABC7's Mike Shumann reacts to Dwight Clark's ALS news
Other teammates have responded to Clark's news.
In a statement, Joe Montana said, "Jennifer and I are saddened by the diagnosis of Dwight. This is a difficult time for Dwight, Kelly and all of us who love him."
And Jerry Rice had this to day, "We know him for 'The Catch,' but he's a great individual who cares about people. I love this guy."
Here's Clark's letter in its entirety:
"In September of 2015, I started feeling weakness in my left hand. I was mildly paying attention to it because since my playing days, I've constantly had pain in my neck. I was thinking it was related to some kind of nerve damage because it would just come and go.
After months of tests and treatment, I got some bad news. I was diagnosed with amyotrophic lateral sclerosis.
I have ALS, also known as Lou Gehrig's disease. Those words are still very hard for me to say.
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.
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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. I visited six neurologists
and three ALS specialists. I also was treated for a B12 deficiency, which sometimes can mirror the symptoms of this debilitating disease.
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. I can't run, play golf or walk any distances. Picking up anything over 30 pounds is a chore. The one piece of good news is that the disease seems to be progressing more slowly than in some patients.
I've been asked if playing football caused this. I don't know for sure. But I certainly suspect it did. 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.
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. 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 get the same pep talk from the Boss, Eddie D. His support has been incredible. So rest assured, I know I'm not alone in this fight.
Every single one of my 49ers teammates that has contacted me has said whatever I need, anytime I need it, they will help. That's just the kind of guys they are. They were so giving as players and now they are the same as friends.
I can't thank my teammates and friends enough for their support. Mr. D always treated us like family and that family is still together. I also want to thank all the great 49ers fans. Your support over the last 35 years has allowed me to remain connected to you. Rarely does a day go by when I'm not asked about 'the Catch,' when we were able to get past the Cowboys and go on to win our first Super Bowl.
I'm not having a press conference or doing any interviews. That time will come. Right now, I've got work to do. 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. 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."