22nd October 2017, 20:17 | Leeds
The 17-year-old Huddersfield-born centre became the first player born in the 21st century to feature in a top-flight game when he made his Rhinos debut at Wigan Warriors in July.
Newman had a year left on his academy contract but has been upgraded to a Super League deal that runs to the end of 2020 and has been promised a squad number for 2018.
Leeds chief executive Gary Hetherington said: "We are pleased to have secured a player of Harry's potential to the club on a long-term basis.
"We have an exciting crop of young players at the club now with Harry joining Jack Walker and Alex Sutcliffe in the first-team squad next season and players like Cameron Smith, Mikolaj Oledzki and Josh Jordan-Roberts all being a year further on in their development from their time playing in the Championship last season."
Newman, a student at Huddersfield New College, said: "I am very proud to have signed a first-team contract with Leeds Rhinos. This season it has been inspirational to see how well Jack Walker has done and Leeds have a history of giving young players a chance if they prove they are good enough.
"I am really looking forward to doing a full pre-season with the first-team players. Training alongside people like Kallum Watkins, Joel Moon and Liam Sutcliffe will be massive for my development in the next few years and hopefully I can make the most of any opportunities that come my way."