The South African side have a partnership with the Aviva Premiership champions, and their need for a loosehead prop ahead of the Super Rugby season has seen Sarries allow the 32-year-old to return to his home city of Cape Town to be part of the Stormers' squad.
Springbok cap Carstens has been with Saracens since the start of the 2010/11 season, making 29 appearances in total.
"We have a partnership with Western Province rugby, and any partnership must work both ways," director of rugby Mark McCall said in a Saracens statement.
"The Stormers urgently need a top-class international loosehead prop for the forthcoming Super Rugby season, the move makes sense for the player and so we have agreed to the release.
"Deon has made a magnificent impact at Saracens over the past 18 months, and his departure presents us with some difficult mid-season challenges, but we wish him, his wife and young daughter all the best in Cape Town.
"Since this is the right decision for Deon and his family, it must be the right decision for us."
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