The Magpies revealed as part of their retained list at the end of last month that both players had been offered new deals to remain at St James' Park.
Teammates Matt Ritchie and Mark Gillespie both had one-year options in their contracts triggered by Newcastle in June, and the club have now announced that Karius and Dummett have also extended their deals until the summer of 2024.
Karius posted a brief message on Instagram about his new contract, which read: "The mission goes on, happy to be a part of it."
Dummett also took to social media to express his pride after penning fresh terms with the club, adding: "Immensely proud to have extended my stay at this incredible club into my 11th year.
"To have witnessed what we have achieved in such a short space of time, not only as a player, but as a fan, has been amazing to see and be a part of. I can't wait to see what this season brings."
Karius is yet to make a single Premier League appearance for Newcastle since joining the club on a free transfer in September last year, with Nick Pope regarded as the undisputed number one under Eddie Howe.
However, the 30-year-old former Liverpool goalkeeper started between the sticks in last season's 2-0 EFL Cup final defeat to Manchester United at Wembley Stadium.
Karius, who is expected to provide backup for Pope once again next season, has only made six appearances at club level in all competitions since the summer of 2020, including five outings during a loan spell with Turkish club Besiktas in the 2020-21 campaign.
As for Dummett, the 31-year-old defender is Newcastle's longest-serving player within the current first-team squad having joined the academy at the age of nine.
Dummett has made a total of 205 appearances for the Magpies across all competitions, although he only featured once in all competitions last season - playing the full 90 minutes in a 2-1 EFL Cup second-round win over Tranmere Rovers back in August 2022.
Both Karius and Dummett were among the first-team players who reported back to training at Newcastle's Benton base for the club's first day of pre-season on Sunday.
The Magpies are scheduled to face Gateshead in their first friendly fixture next Saturday, and their pre-season preparations will continue with encounters against Rangers, Fiorentina and Villarreal between the end of this month and the beginning of August.