The 24-year-old's contract in Rotterdam was up at the end of the season, and he had a public disagreement with Feyenoord coach Robin van Persie last week after being left out of the starting lineup for a derby loss to Sparta Rotterdam.
"Quinten Timber has established himself as one of the most promising midfielders of his generation," Marseille said in a statement.
Timber was part of the Feyenoord side which lifted the Eredivisie title in 2023 under current Liverpool coach Arne Slot and made 125 appearances in total for the club, scoring 21 goals.
He has made eight appearances for the Dutch national team since making his debut in 2024 and is in contention for a place in their 2026 World Cup squad.
Marseille sit third in the French top flight, eight points behind leaders Lens, and are battling for a place in the Champions League knockout playoff round.