The veteran centre-back's contract at the Stade Velodrome expired at the end of last season and Nice have moved to snap up the Senegal international.
Diawara made 118 appearances in five seasons for Marseille, winning the Ligue 1 title with the club in 2010.
That league triumph came one year after he had achieved the same feat with Bordeaux, whom he had joined from Charlton Athletic in 2007.
Aside from his two-year stint in England with Charlton, Diawara has spent his entire professional career in France, previously representing Le Havre and Sochaux.
Nice finished 17th in Ligue 1 last season, narrowly avoiding relegation.
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