According to Globo.com, Besiktas formalized its interest in and a proposal for Martinelli to Fluminense at some point between the beginning of the year and the present. The highest amount offered by one of the clubs was 14 million euros. All proposals were made before Martinelli's call-up to the Brazil national team. Fluminense currently considers Martinelli to be non-negotiable because of his importance to the team and the current decisive stage of the season. The club anticipates that new proposals could emerge in the next transfer window, particularly following the call-up.

Mateus Martinelli is in open talks with Besiktas. According to Turkish media, Besiktas have advanced their contacts with the player, but an agreement has not yet been finalized.

The Turkish outlet 'Fotospor' reports that Besiktas is in contact with Fluminense to acquire Martinelli.

Beşiktaş, Olympiacos, West Ham, and teams from the Gulf submitted offers of approximately €12,000,000 for 24-year-old Brazilian defensive midfielder Matheus Martinelli. Fluminense rejected all of these proposals, as the club values the player at a minimum of €20,000,000. Martinelli is under contract until 2027, and Fluminense is seeking to extend his deal until 2030. According to Transfermarkt, his market value is €14.00m. For the 2025/2026 season, he has made 3 appearances and 1 assist in the Campeonato Carioca, and 1 appearance in Série A.