The 33-year-old, the most capped skipper and forward in Springbok’s history, will join upon the conclusion of October’s World Cup in New Zealand.
“I am delighted to have signed for a club widely renowned for their ferocious work ethic,” said Smit. “It is becoming a very special rugby club, combining a commitment to develop English players with a clear South African flavour.”
Smit’s fellow South African, Schalk Brits, has been a huge success in his preferred position of hooker since arriving at Vicarage Road in 2009.
Brits’ all-action style earned him the man-of-the-match award as Saracens clinched their first Premiership title in May at the expense of Leicester and he is unlikely to be dislodged easily from Mike McCall’s first team.
“We are constantly looking to strengthen, and although Schalk Brits is one of the best players in the world in his position, we were aware that we needed more depth in that position,” added chief executive Edward Griffiths.
“John Smit became available and not only is he an outstanding player, but also an outstanding individual and leader.
“We are an English club with a strong South African flavour and he fits well with that.”