The defender arrived in the North East on Sunday, and after productive talks has been confirmed as the Black Cats' second acquisition of the January transfer window, after Alfred N'Diaye joined from Bursaspor last week.
Mangane is currently part-way through a three-year contract with Saudi Arabian side Al-Hilal, and the deal means that the Senegal international will stay at the Stadium of Light on loan until the end of the season, with a view to a permanent move in the summer.
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"He will improve competition for places and we are delighted to welcome him to the club."
Mangane, linked with both Arsenal and West Ham last summer, made over 100 appearances for Rennes between 2008 and 2012, and has made six appearances for Senegal, scoring once.