Boss Alan Pardew revealed on Thursday that the 22-year-old full-back had been offered a new deal and the club confirmed on Saturday morning that he has agreed terms on a contract that could rise to six years in length.
The Wales Under-21 international's previous contract was due to expire at the end of the season, so Newcastle acted swiftly to retain his services.
Dummett broke into the first team this season and scored his first goal for Newcastle in the draw with Liverpool last month.
And the Tyneside-born youngster is happy to have committed his future to the club.
"I am delighted to secure my long-term future with Newcastle United," Dummett told the club's official website.
"Like so many local lads it has been my boyhood dream to play for this club and now I am looking forward to repaying their faith in me and working hard to be the best possible player I can be.
"This is a great moment for my family and so many others who have supported me over the years, and I would like to thank them all."
Dummett will play no part in Saturday's Premier League clash with Chelsea at St James' Park due to a hamstring injury.