— BBC Sport (@BBCSport) July 28, 2015
He has rejected a contract from Premier League club West Bromwich in order to join the French outfit.
Abou Diaby will play on a pay per play basis at Stade Velodrome.
The Frenchman was a free agent after he was released by Arsenal during the summer following a succession of injury-ravaged seasons with the Gunners, with the 29-year-old suffering from as many as 40 separate injuries during nine years with the club.
The France international played 180 games for Arsenal, but was restricted to just one first-team appearance last term, against Southampton in the third round of the Capital One Cup.
Diaby moved to north London from Auxerre in 2006 and was the longest-serving player in the Arsenal squad.
West Brom were favouritres to sign the French star few days ago but Marseille launched an eleventh hour bid for the former Arsenal man.