When the 89-Test veteran saw a doctor last month, he was given just three years to live.
But the 40-year-old is refusing to give up on life despite the shocking news.
"I will fight this thing. Even if it's the last thing I do," van der Westhuizen, who played in all three Tests against the Lions in 1997, told the Rapport newspaper in his homeland.
"I'm fighting this illness with my faith. And my faith is strong."
Van der Westhuizen was widely regarded as the world's best No9 for large parts of a 10-year international career that spanned three World Cups and included the victorious 1995 campaign on South African soil.
But the Springboks' joint-record try scorer has experienced some difficult times in recent years, including suffering a heart attack shortly after watching the second Test on the 2009 Lions tour.
The latest setback came to a head while he was on a recent break with his children in Sun City, although he admitted the first indications that something was wrong actually came about back in December.
"I struggled to move my hand, but I just left it," added van der Westhuizen.
"It's typical; I thought it was an old sports injury. As time passed my speech became impaired. When I spoke to people, they would say, 'Joost, are you drunk?'
"After my doctor told me I have motor neurone disease, he said that doctors didn't know how to treat it.
"I became a blank. I hardly remember anything from that day."
Van der Westhuizen is currently waiting for the results of a second opinion from a Johannesburg neurologist.