Fellaini signed for United from Everton back in 2013 when David Moyes took over from Sir Alex Ferguson.
"I am pleased to be continuing my journey as a Manchester United player", he told United's website.
Fellaini's contract situation had divided United supporters, a section of whom did not want the Belgian to be handed a new deal due to his inconsistent performances, but head coach José Mourinho has always backed the towering midfielder.
Courtois said: "Well I think Marouane is a player that often is criticised by a lot of fans, but I think he's a very useful player".
The 30-year-old started as Belgium beat England at the World Cup on Thursday, after which he reiterated that news about his future was imminent.
Kroos has been one of the best midfielders in the world over the past five years and he's helped Madrid win a whole host of trophies including 3 Champions League's and 1 La Liga title since joining the club from Bayern Munich in 2014.
The Belgian midfielder, who featured in Thursday night's win over England in the ongoing World Cup in Russian Federation, thanked Manchester United manager Jose Mourinho "for the faith he has" in him.
After the contract was official, Mourinho said the following: "I am very happy Marouane is staying with us".