The new deal includes the option to extend his stay by a further year.
The 30-year-old Belgium international joined United from Everton in 2013 and has made 156 appearances and scored 20 goals, winning the the FA Cup, League Cup and Europa League.
“I am pleased to be continuing my journey as a Manchester United player,” Fellaini told manutd.com.
“I made this decision because I am very happy here. Also, I feel like this team, under Jose, still has a lot we want to achieve.”
Fellaini has become something of a favourite for Jose Mourinho and, despite recent speculation that the midfielder was considering leaving, the United boss was convinced he would extend his stay.
"I am very happy Marouane is staying with us," Mourinho said. "I always believed in his desire to stay with the club and I am delighted that he has signed a new contract."
Fellaini became David Moyes' first significant signing after he replaced Sir Alex Ferguson.
The midfielder endured two spells out of the team last season because of a knee injury that eventually required surgery.
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