Fellaini's previous deal at Old Trafford was set to expire on Saturday, but the club announced that the 30-year-old would remain a part of manager Jose Mourinho's squad.
United released a statement with quotes from the player, who said:
"I am pleased to be continuing my journey as a Manchester United player. 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.
"I would like to say a special thank you to Jose for the faith he has always shown in me. My focus now is on the World Cup and I am looking forward to a successful season ahead."
Fellaini has made 156 appearances for the Red Devils since joining from Everton in 2013 when he was recruited by his former Toffees manager David Moyes.
There was speculation over Fellaini's future for much of this season and rumours regarding the terms upon which he would remain, though BBC Sport's Simon Stone said the club had not budged in negotiations:
Fellaini is on international duty with Belgium at the 2018 FIFA World Cup in Russia and contributed a starring display in their 1-0 win over England on Thursday to top Group G, his first start of the competition.
Samuel Luckhurst of the Manchester Evening News appeared to suggest the move was an unwise investment on United's part, offering a new deal to a midfielder signed during something of a crisis period for the club:
But Fellaini has also been with the club during their rejuvenation since the departure of Sir Alex Ferguson from the helm, winning the FA Cup (2016), UEFA Europa League and the EFL Cup (both 2017).
ESPN's Alex Shaw joined those criticising the club's decision, however, and said this was an opportunity for the club to cut ties and evolve their midfield:
Mourinho gave Fellaini just seven starts in all competitions last season as a knee injury limited the player's impact, which in part fuelled rumours that he was set to leave the Theatre of Dreams.
Fellaini spent six seasons at Goodison Park but will eclipse that number and make United the most prominent club on his resume if he sees out the entirety of his new deal.