Fellaini penned an initial four-year contract at Old Trafford when he moved from Everton in the summer of 2013, but United had an option to extend that by a further 12 months.
Boss Jose Mourinho has opted to activate that clause, despite question marks over Fellaini’s future at the club and boos from some supporters during United’s EFL Cup win over Hull in midweek
The Belgian tellingly ran to Mourinho to celebrate his goal in that game after a difficult period in which he conceded a late penalty in last month’s draw at Everton.
"He has a very strong mentality and has coped well,” said Mourinho earlier this week.
"He knows he is a very important player for me."