The 33-year-old England international has made 266 appearances since joining United in 2019 for 80 million pounds ($106 million), then a record fee for a centre back, helping them win the FA Cup and League Cup.
"Representing Manchester United is the ultimate honour," Maguire said. "I am delighted to extend my journey at this incredible club to at least eight seasons and continue to play in front of our special supporters to create more amazing moments together."
Maguire has re-established himself in the team under interim manager Michael Carrick after a difficult period in which he lost the captaincy and his regular starting place, helping them climb to third in the league table.
"Harry represents the mentality and resilience to perform for Manchester United," United Director of Football Jason Wilcox said. "He is the ultimate professional who brings invaluable leadership to our young, ambitious squad."