The Belgium international had 18 months remaining on his existing deal, but has now agreed to stay with the Toffees until 2016.
Fellaini, who has made 14 appearances in all competitions this season, became Everton's record signing when he joined the club for €17.5 million from Standard Liege in 2008.
"We are all delighted we have secured the services of Marouane for the long term," Everton chief executive Robert Elstone told the club's official website.
"As everyone is aware these negotiations have been ongoing for some time but both parties remained positive that we would reach agreement.
"Marouane has been a hugely influential player for Everton since his arrival in 2008 and we hope that remains to be the case for the next five years."
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