The 24-year-old arrives after an impressive spell in Ligue 1, where he scored 16 league goals last season to finish joint-second in the Golden Boot race behind Rennes striker Esteban Lepaul.
Greenwood leaves Marseille having registered 48 goals in 81 appearances across all competitions since joining from Manchester United in 2024.
The Old Trafford giants are expected to benefit from a sizeable sell-on clause included in the transfer agreement.
Speaking after completing the move, Greenwood thanked Fenerbahce supporters for their welcome, saying he was delighted to join the Turkish giants as they target domestic and European success under their new leadership.
"Hello Fenerbahce family, thank you so much for the support you have shown me. I am very happy to be here."