The 21-year-old attacking midfielder was named Marseille's player of the season for 2010-11 and was crowned BBC African Footballer of the Year on Friday.
"OM will depend on him in the years to come," said Marseille coach Didier Deschamps. "It's a very good thing that he's extended (his contract)."
Ayew, the son of former Marseille star Abedi Pele, is a graduate of the OM academy and has been named in the Ghana squad for the Africa Cup of Nations in January and February.
After loan spells at Lorient and Arles-Avignon, Ayew broke into the Marseille first team last season and played 37 matches, scoring 11 goals.
He has scored six goals in 16 matches this season, occasionally lining up alongside his younger brother, Jordan.
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