The England defender, 33, was due to become a free agent in the summer but has extended his Red Devils contract by one year, with an option for a further 12 months.
Maguire, also a target for Napoli, said: “The determination throughout the whole club to fight for major trophies is clear for everyone to see and I am confident that our best moments together remain ahead of us.
“You can feel the ambition and potential of this exciting squad.”
Maguire, who arrived for £85million in 2019, has started all ten of interim boss Michael Carrick’s games in charge and emerged as one of United’s most pivotal players.
His form led to interest from MLS side Inter Miami, co-owned by United and England legend Beckham, and Italian champions Napoli.
The 65-cap star was recalled by Three Lions boss Thomas Tuchel in the last international break — following an 18-month absence — ahead of this summer’s World Cup.
SunSport understands Tuchel was regularly in touch with Maguire earlier this season when he missed nine club matches with a thigh injury.
Maguire, who wants to complete his coaching badges at United, added:
“Representing Manchester United is the ultimate honour.
“It is a responsibility that makes myself and my family proud every single day.
“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.”
United accepted a £30m offer from West Ham for Maguire in August 2023 after he was stripped of the captaincy by former manager Erik ten Hag.
But the club’s failure to make up the shortfall in Maguire’s £200,000-a-week wages caused the deal to collapse.
The new deal completes a remarkable turnaround for the Sheffield-born defender, who has made 266 appearances for the Red Devils.
Director of football Jason Wilcox added:
“Harry represents the mentality and resilience required to perform for Manchester United.
“He is the ultimate professional who brings invaluable experience and leadership to our young, ambitious squad.
“Harry, like everyone at the club, is completely determined to help Manchester United to achieve regular and sustained success.”
United signed Maguire from Leicester for what remains a world-record fee for a defender.
He was made club captain by then-manager Ole Gunnar Solskjaer in January 2020 after Ashley Young left for Inter Milan and led United for three-and-a-half years.
Maguire will be suspended for the Premier League visit of Leeds on Monday after he was sent off in the 2-2 draw at Bournemouth last month.
The FA has also charged Maguire with improper conduct after a run-in with the fourth official at the Vitality Stadium.