The deal agreed is worth €40million.
The contract is 4years and runs until the summer of 2026.
The Senegalese international had earlier agreed personal terms with the Club World Champions.
The move will see the nonsense defender earn a net salary of €10million.
Fans are already calling for the Blues to announce the signing and unveil the defender along with recent acquisition, Raheem Sterling, from rivals, Manchester City.
Both signings are now expected to join up with their new teammates in the tour of the U.S.A