The Comoros international has concluded his transfer to the Ligue 1 giants after agreeing terms in the south of France
The Comoros stopper has risen up the French game after joining lower-league side Marignane in 2007. He moved to Sedan in 2009 and played around 100 games for the side.
However, L'OM agreed an undisclosed fee with Sedan, and have now finalised the deal.
New boss Elie Baup has had the perfect start to his Marseille career, winning all three of his matches to date, and will hope Abdallah can be included in his squad for Sunday's clash with Rennes.