The Foundation asked that Joost and his family's privacy be respected.
Van der Westhuizen, who has been fighting motor neuron disease since 2011, played 89 Tests for the Springboks between 1993 and 2003.
He scored a then-Springbok record 38 tries and is widely regarded as one of the game's finest-ever scrumhalves.
The J9 foundation confirmed the news via their Facebook page:
"The family and friends of Joost ask that you all pray for Joost. He was taken to hospital earlier today and is in a critical condition.
We ask that you all please respect the families time at the moment.
When we have updates we will let you know for now we ask that we all bow our heads in prayer."
Latest update on Joost:
Latest update on Joost:— J9 Foundation (@J9_FoundationUK) February 4, 2017
The family and friends of Joost ask that you all pray for Joost. He was taken to... https://t.co/eZY6gBQBkv