The one-year extension will tie the Belgian midfielder to Old Trafford until at least the summer of 2018.
The 29-year-old scored his first goal of the season during the win against Hull City on Tuesday.
Fellaini joined United from Everton on the final day of the summer transfer window in 2013.
A good week for @Fellaini – a goal against Hull followed by an #MUFC contract extension until 2018! More: https://t.co/IrP9pVvqmb pic.twitter.com/lOQEk8YcIf
— Manchester United (@ManUtd) January 12, 2017