South Africa captain John Smit will join Saracens in October after the conclusion of the 2011 World Cup.
The 33-year-old is the most capped skipper and forward in Springboks history, having featured in 102 Tests.
The hooker played for South African Super Rugby outfit Sharks in 2011, helping them to the quarter-finals.
"I am delighted to have signed for a club widely renowned for their ferocious work ethic and spirit on the field," he told the club website.
"It is becoming a very special rugby club, combining a cast-iron commitment to develop young English players with a clear South African flavour.
"I'm looking forward to rejoining former team-mates like Deon Carstens and Brad Barritt, and also to working with players like Jamie George, last year's England Under-20 hooker."
Director of rugby Mark McCall added: "We are absolutely thrilled to have signed one of the most prominent and admired players in world rugby.
"John is an outstanding leader and hooker, and I am certain he, Roxy and their children will all thrive in our environment."
The 2007 World Cup winner, who has also featured at tight-head prop for his country, has had one previous spell outside of his homeland, playing for French side Clermont in 2007-08.
Smit's fellow South African, Schalk Brits, has been a huge success at hooker since arriving at Vicarage Road in 2009.
Brits' dynamic style earned him the man-of-the-match award as Saracens clinched their first Premiership title in May and he is unlikely to be dislodged easily from 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," said chief executive Edward Griffiths.
"John Smit became available and not only is he an outstanding player, but also an outstanding individual and leader."
Scrum-halves Richard Wigglesworth and Neil de Kock rotated in the first-team position during the last campaign, with De Kock winning a coin toss to decide who would start the final against Leicester.
Griffiths refused to be drawn on whether a similar arrangement would exist at hooker this season, but insisted Brits would stay as Saracens "need a squad, not a team" for successful Premiership and Heineken Cup campaigns.
The club has been majority owned by South African billionaire Johann Rupert since January 2009 and Griffiths was hopeful Smit's arrival would help attract new fans.
"Our player recruitment is always done from a rugby perspective, but having a World Cup-winning captain will obviously do no harm," he added.
"We are an English club with a strong South African flavour and he fits well with that."