The former Nancy, Queens Park Rangers and Al Ittihad man is expected to help the Paris side return to Ligue 2 after their relegation last season.
The length of the 28-year-old's deal with Red Star was not revealed.
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Diakite had been a free agent after leaving Rangers last year. His stay at QPR was hampered by injuries which subsequently saw him loaned out to Watford and Al Ittihad in Saudi Arabia.