Itoje, who has won 29 caps for England and the British and Irish Lions, has helped Saracens win back-to-back Champions Cups and three Premiership titles.
"I'm really pleased to be staying on at Saracens," Itoje said in a club statement.
"I've been at the club for 10 years now so I am really excited to see what the future holds for this great team."
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The 24-year-old forward has made more than 100 appearances for Saracens since making his senior debut in 2013.
(Reporting by Toby Davis; Editing by Greg Stutchbury)
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