Lascelles was handed the armband by Rafa Benítez ahead of last season’s Championship-winning campaign, and has already headed two winning goals for the Magpies in their first season back in the Premier League.
Born in Derby, the defender linked up with Newcastle from Nottingham Forest in 2014, spending a further year at the City Ground on loan before returning to Tyneside ahead of the 2015/16 campaign.
So far, he has made 75 appearances for United in all competitions, finding the net seven times.
Speaking about his new deal, Lascelles said: “I’m over the moon. Since I came to Newcastle, I’ve been happy; I love the city, the football and the fans. Everything about it, I enjoy so I’m happy to be tied down for the next six years here.
“We’ve got a group of ambitious lads, the fans are excellent and the manager has got trust in me, so I’m really happy here and I want to take the club as high as it can go.”
Boss Benítez added: “As soon as I made the decision to make Jamaal the captain of Newcastle United, he rose to the challenge and he has developed as a player and a character.
“He has proven himself to be a very good leader and a good role model too.
“He has also improved as a player, responding well to our advice and guidance and getting better with every game.
“I would like to congratulate him on his new contract with the club, it is very well deserved.”