The 21-year-old Wahi becomes Marseille’s ninth new recruit since the end of last season, joining for a reported fee of 25 million euros ($27.4 million).
That figure is 10 million euros less than what Lens paid for him a year ago when he moved from Montpellier.
Wahi scored 12 goals in 37 appearances in all competitions for Lens, hitting the winner in a memorable Champions League victory over Arsenal.
But his return of nine league goals was well down on the 19 he scored for Montpellier in the 2022-23 season.
The France Under-21 international joins a list of new arrivals at Marseille that includes Mason Greenwood, Pierre-Emile Hojbjerg and Lilian Brassier, who helped Brest to a historic third-place finish last term.
Marseille missed out on European qualification after coming eighth.
Roberto De Zerbi took over as coach in June and said there would be plenty of changes to the team ahead of the new Ligue 1 season, which begins this weekend.
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