The 31-year-old has been a free agent since leaving Turkish club Genclerbirligi, where he made 27 appearances last season, scoring six goals and providing two assists.
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A product of the SM Caen academy, Niang made his professional debut with the club and has since represented 15 other teams, including AC Milan, Watford and Wydad AC.
He returns to France with significant experience, having featured for Montpellier HSC, Stade Rennais, Girondins de Bordeaux and AJ Auxerre.
Niang has made more than 160 Ligue 1 appearances and scored around 40 goals. His arrival strengthens a Dunkerque squad that recently added 2018 World Cup winner Steven Nzonzi.