Fenerbahce say the deal is worth €39m, having previously indicated the total package would reach €45m. The move was announced on the same evening as the World Cup semi-final between France and Spain.
In an official statement, the Istanbul club set out the terms of the transfer in detail.
"Our club has reached an agreement with the club and the player, Mason Greenwood, regarding his transfer. The 24-year-old player has signed a four-year contract. The total transfer fee of €39,000,000 to Olympique de Marseille will be paid to Olympique de Marseille in three equal instalments over three years.
"In addition, the solidarity contribution, in accordance with FIFA regulations, will be covered by our club. We welcome Mason Greenwood to our family and wish him great success and titles with Fenerbahce."
Greenwood arrives at the Sukru Saracoglu Stadium having finished as joint-second in the Ligue 1 golden boot race last season, scoring 16 goals behind Rennes forward Esteban Lepaul.
The forward's arrival follows public interest in the deal from both of Fenerbahce's presidential candidates ahead of June's club elections, with Aziz Yildirim, who went on to be elected, and rival Hakan Safi both having made their wish to sign the striker known before the vote.
"Hello Fenerbahce family, thank you so much for the support you have shown me. I am very happy to be here," Greenwood said following the announcement.
Greenwood's exit from Marseille comes after reports in France of friction between the player and the club's hierarchy in recent months, though he remained one of the squad's most reliable goal threats throughout.
Manchester United, who sold Greenwood to Marseille in 2024, are understood to be in line for a percentage of the fee under a sell-on clause included in that original deal, with Getafe also due a smaller share having sanctioned his loan spell before the permanent move to France.
The transfer brings an end to Greenwood's spell at Marseille, where he established himself as one of Ligue 1's most productive forwards after joining the club from Manchester United in the summer of 2024, following a loan spell at Getafe. Fenerbahce will be hoping his goalscoring form transfers to the Turkish Super Lig as the club look to challenge for the title next season.