The 23-year-old centre-back joined Sunderland permanently from PSV Eindhoven in 2023 and made 18 senior appearances during his debut campaign.
A product of PSV’s academy, Seelt also featured 60 times for Jong PSV before making his first-team debut against PEC Zwolle in May 2022.
He began Sunderland’s 2025-26 Premier League campaign against West Ham before moving to Bundesliga side Wolfsburg on loan, where he made 10 appearances.
The Dutch defender will now continue his development with Swansea, gaining valuable Championship experience while remaining under contract with Sunderland until 2030.
“Jenson has earned his new contract based on his performances during his time at the club,” said Florent Ghisolfi.
“We are looking for him to continue to develop his game whilst on loan at Swansea this season.
“By signing the contract before he completes the loan move, it demonstrates that we see him as a player who is very much part of Sunderland’s plans in the future, which is what we’re all working towards.”