51-year-old Johnson -- who won 4 gold medals in 3 Olympics -- tells TMZ Sports he suffered a stroke on the right side of his brain following a workout in Sept. 2018.
At the time, Johnson said he felt discomfort in his left leg, numbness in his left arm and quickly began to lose balance. So, he rushed to the emergency room to get checked out.
"By the time I got to the hospital, I couldn't walk anymore. I couldn't stand on my left side," Johnson says.
"But, fortunately, I got there quickly and was able to get treated by a team of specialists."
Johnson spent the next several months working his ass off in several rehabilitation programs including physical and occupational therapy ... and says he approached rehab like he was training for the Olympics.
"[Rehabilitating was] a little more difficult on the one hand because no one's standing there clapping there for me when I finish my workout now," Johnson said.
"But, at the same time, this is my livelihood that was at stake."
Ultimately, Johnson was able to get back to the high level of physical fitness he was at before the stroke -- and now he's on a mission to spread awareness along with the American Stroke Association.