Former England skipper Borthwick, 31, led Sarries to their maiden Premiership success when they beat Leicester in May's Twickenham final.
Saracens have not revealed the length of the new deal for the former Bath lock, but it follows hot on the heels of director of rugby Mark McCall and his entire coaching team committing themselves to the club until 2014.
"People talk about Saracens as a special club, and they are right in many ways," said Borthwick.
"There is a fantastic group of people at Saracens who work incredibly hard for each other to continue to build this great club.
"We have a long way to go in many respects, and I'm looking forward to the journey."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
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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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