The Senegal international has struck a deal worth around £8million a year with a further £1million available in bonuses and is following Raheem Sterling enters Stamford Bridge this summer.
Chelsea is also watching Jules Kunde came to a stop after their pursuit of Nathan Ake as Thomas Tuchel welcomes in the first signing of a spree under new owners Todd Boehly and Jose Feliciano and Behdad Egbahli in Clearlake Capital.
“We are very happy that he has signed and has now joined our team,” said Tuchel.
!It’s the second signing of the summer. He is the second acquisition of the summer. He is a strong player, a strong personality and a lot of experience. In short, a perfect addition to the team.”
Koulibaly and possibly Kounde will replace Antonio Rudiger and Andreas Christensen, who came to Real Madrid and Barcelona respectively after their contracts expired.
Presnel Kimpembe. from Paris Saint Germain is also a target with Tuchel contemplating a third defensive strike amid doubts about Cesar Azpilicueta’s future.
The 32-year-old has agreed a bumper contract with Barcelona, although Chelsea’s valuation of around € £7million for their club captain could complicate matters.
Tuchel’s squad will play their first friendly of the season on Saturday against Club America in Las Vegas, before taking on Charlotte FC in Charlotte and Arsenal in Orlando on their US tour.