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.
"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.
