The Haiti international, who helped fire Sunderland to promotion last season and found the net in the play-off semi-final win at Coventry City, described the extension as a natural next step in a relationship that has grown since his arrival from Zenit Saint Petersburg.
"I feel proud and happy to continue my journey here," Isidor said. "Everyone knows how proud I am to play here with this shirt and for this city, so thank you to the club for extending my contract. I love everything here — the people, the fans, the stadium, the city.
"The way the supporters welcomed me was amazing, and I want to learn even more about the culture. It's a perfect match for me here and I always try to repay the supporters."
The move comes off the back of a landmark summer for Isidor, who helped Haiti reach the World Cup for the first time since 1974.
"The World Cup was amazing," he said. "I'm always proud to represent my country and it was a privilege. We didn't go through, but the image we left on and off the pitch was amazing. People will not forget that Haiti played at the 2026 World Cup."
With his long-term future secured, Isidor is now focused on the campaign ahead, which will bring Premier League, Europa League and domestic cup football to Sunderland for the first time in years.
"I hope this year I can continue to develop with all the European games, cup games and Premier League," he said. "I want to score more this year, and that's why I think the club believes in me. It's an amazing time to be a Sunderland player."
The new deal also draws a line under recent transfer speculation, with West Ham having been strongly linked with Isidor earlier in the window as they searched for attacking reinforcements.
Since arriving on an initial loan in 2024 before Sunderland exercised the option to sign him permanently, Isidor has made 82 appearances for the club, scoring 19 goals, and his commitment to a new long-term deal underlines how central he has become to Régis Le Bris's plans as the club prepares for life back in the top flight.