The 25-year-old will make the move from the Sharks - with club having been keen on bolstering their second-row options following Smith's decision to join the New York Jets.
And Hargreaves, who has been capped four times but was overlooked for the World Cup, is relishing the chance to prove himself in the Premiership.
Hargreaves told the club's official site: "Everybody in South African rugby knows there is something special happening at Saracens, and I can't wait to get stuck in and make a contribution to the continuing development of the club.
"This is a fantastic opportunity for me at a crucial stage of my rugby career."
Saracens director of rugby Mark McCall is expecting big things from the powerhouse lock.
He added: "Alistair has a fantastic pedigree in SA rugby and we are confident he will flourish in our environment."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
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The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
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The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.
