The 26-year-old Free State Cheetahs player was included earlier this year in a provisional Springboks training squad for upcoming internationals and joins on a two year contract.
Standing 6ft 5in and weighing 18st van der Westhuizen has started all 12 of the Cheetahs matches in the Super 15 competition this year.
In a statement released by Edinburgh van der Westhuizen, who will arrive in October subject to a visa being granted, says: “I want to test myself against the top players in Europe so when I found out Edinburgh Rugby were looking for a player with my skill-set, I was desperate to sign.
“I like the pace the boys play with, it’s very similar to a Super Rugby style which gives me a lot of confidence that I’ll fit in well.
“I run the set-piece for the Cheetahs but, when the ball’s in play, I like to be in the centre of the park getting stuck into the physical side of the game and hitting rucks.
“I’m really looking forward to joining the club. They’ve some real talent joining the squad and some quality Scottish players re-signed too so I think it’s the ideal time to be part of a really exciting squad.”
Edinburgh coach Michael Bradley said: “Over the course of this season we have been putting the right building blocks in place to tackle what is arguably the most eagerly anticipated season in the club’s history.
“Reaching the Heineken Cup semi-final set the benchmark for our players’ performance so we have recruited and retained a number of key players to bring consistency to our season and set a new standard for Edinburgh Rugby.
“The increased investment in our squad from Scottish Rugby will not only intensify the competition for places but means we will be on a much stronger footing to compete on all fronts, all season.”
Meanwhile, Cheetahs second row George Earle, linked with a move to Edinburgh, today joined Scarlets.