Speaking to United's club channels about his new contract, Maguire said: "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.
"You can feel the ambition and potential of this exciting squad. 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."