The pair, who came through the club's academy, have put pen to paper on deals that will run until 2019.
England international Itoje, who made his debut in May 2014, helped the team to LV= Cup triumph and the Premiership title last season, while Kruis has made 118 appearances for the club.
"I'm extremely lucky to call Sarries home and I'm delighted to extend my contract for three more years," Itoje told the club's official website. "It was an easy decision from the outset, as I've really enjoyed my rugby so far and I'm incredibly grateful for the help and guidance I have received since arriving at the club as a 14-year-old.
"The club is hugely ambitious and has been competing at the highest level for some time. As a player you want to be playing in the big games, and once again we are well placed in both the Aviva Premiership and European Champions Cup this season. It's a special club to play for and I'm really happy to be staying."
Kruis said: "Saracens is a club that is in my heart, and I didn't hesitate to sign a new deal to stay here. I owe so much to the club and I am really looking forward to my future at Allianz Park.
"What's really important to me is that as a young group of players we are all committing our futures to the club together. I'm playing alongside my best mates and working alongside them day-in, day-out and that means a lot to me. We've already achieved so much as group of players and there's still so much more to come from us, I'm delighted to be staying here."