The 29-year-old forward is set to fill the vacancy at Vicarage Road created by England World Cup winner Richard Hill's retirement.
The highly-rated Van Heerden has won 14 caps for South Africa, including six appearances during their World Cup-winning campaign last autumn, two of those against England at Stade de France.
The Pretoria-based Bulls say Van Heerden will link up with Saracens in November after completing an existing two-year contract.
Bulls' business and communication manager Ian Schwartz paid a glowing tribute to the Johannesburg-born flanker.
"When he signed with us at the end of 2006, he indicated that he wanted to play for the Bulls for two years, after which he would like to further his career in Europe," Schwartz told the Bulls' official website. "We did, however, try to keep Wikus at Loftus Versfeld.
"He has awe-inspiring in 2007, and is one of a handful of players who has a Super 14 as well as a World Cup (winners') medal.
"He continued his good form in the 2008 Super 14, and it will be very difficult for the national selectors to ignore him.
"Wikus is an unbelievable team player, leader, thinker and the perfect example for younger players as to what it means to be professional.
"We wish him all the best for the future, and we know that success with be inevitable at Saracens. Our loss is their gain."
European Cup semi-finalists Saracens have already recruited current England captain Steve Borthwick from Bath for next season, in addition to Wales international forward Michael Owen.