Michael Bradley's side have lost five games in succession including two Heineken Cup reverses at the hands of Saracens and Munster in which they failed to register a single point.
Van der Westhuizen was a cornerstone of the Free State Cheetah's pack and is the third Southern Hemisphere signing, following the arrivals of fellow Cheetahs forward Willem Nel and the former Highlanders centre Ben Atiga from Super Rugby in the summer and should bring some much-need help at set-pieces.
The second row is line to make his debut against Ulster in RaboDirect Pro12 action and says Nel played a part in convincing him that his future lay in Scotland.
"I consider myself a pretty physical player, particularly in the tackle and contact areas, clearing out rucks and so on, while my lineout work is also something I pride myself on. I've spoken to Willem [Nel] since he arrived and he's told me how professional the set-up is and how welcome he's been made to feel. I'm really looking forward to my first match in the black and red of Edinburgh Rugby."
The 26-year-old has missed just 40 minutes of rugby in 30 games, but he played down concerns that might not have enough left in the tank to contribute effectively during the months ahead.
He said: "I've played a lot of rugby this season but I'm keen to meet up with the boys and get into action as soon as possible. It's a new challenge and I'm looking forward to being part of a great club with a good attacking style.
"It's often said that the PRO12 and Heineken Cup are the most physical competitions in the world, so I'm looking forward to seeing how it compares with my experiences in the Southern Hemisphere."