The French club had been tracking the defender for some time and will be delighted to have got their man.
The Senegalese international, who was born in France, began his career in the land of his birth, enjoying spells with Louhans-Cuiseaux and Sochaux before he completed a move to Scotland in 2005.
He has spent the last three seasons with the Jambos and helped them lift the Scottish Cup in 2006.
However, Hearts endured a disappointing 2007/08 campaign, finishing eighth in the table and suffering further off-field upheaval.
Nantes, on the other hand, secured promotion into the French top flight.