Edwards, 48, has caddied for Watson since 1973 and has been on Watsons bag for all eight of his major victories.
According to Watson and others on the tour, Edwards symptoms began appearing last fall. There is no cure for ALS, which is a progressively debilitating disease. Former PGA Tour player Jeff Julian was also diagnosed with ALS in 2001.
Edwards, who is single, plans to continue to work as long as possible.