Lascelles' switch to Leicester - where he's signed a short-term deal until the end of the season - brings to an end his 12-year stay at St James' Park, in which he captained the Magpies from the Championship to the Champions League.
Lascelles made 253 appearances, one of which came in the historic 4-1 Champions League win over PSG at St James' Park. Lascelles wore the captain's armband that night.
He missed last season through injury but Newcastle say he "retained a valuable role within the first-team leadership group, underlined as he lifted the Carabao Cup trophy alongside Bruno Guimarães and Kieran Trippier at Wembley".
In a statement, the club said: "Everybody at Newcastle United wishes Jamaal the very best of luck for the future and thanks him for his outstanding contribution to the club. He will always be welcome at St. James' Park."
Lascelles has made two Premier League appearances this season, the last of which came in September, and he's hoping to play regularly at Leicester.
He said: “I’m just buzzing to get through the door. It’s a great set-up here, a huge club, the facilities are fantastic. I just can’t wait to help out as much as I can, and try and push up the league.
“I’m definitely going to add high standards, professionalism… for me, they’re non-negotiable. It’s the one per cents every day, dragging myself and other people on board and just keeping high standards.
“If you do that in and around the building, on the training pitch, it’ll carry onto the games.
“It’s very exciting [to join Leicester]. I’ve been at Newcastle for a long time, since I was 21. It’s not often that players stay at clubs for that long. I had some great memories there.
“I’ve seen the ups, the downs. It’s a great set-up there, but I’m here and it’s a new chapter. I can’t wait to get started. Like I said, I want to win games, I want to get back to playing football and enjoying playing games, helping out as much as I can.
“That’s all I want to do, play regular football. I’m really looking forward to the challenge."