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.
The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Lions get ready for their matches with this green training jersey. It's what the players wear. Get in the Gear!
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Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
$29.99
The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.
