The Senegal international has put to pen paper on a deal worth around £8m-per-year with a further £1m available in bonuses and follows Raheem Sterling into Stamford Bridge this summer.
Chelsea are also looking at Jules Kounde after their pursuit of Nathan Ake ground to a halt as Thomas Tuchel welcomes in the first signings of a spending spree under new owners Todd Boehly and Jose Feliciano and Behdad Egbahli at Clearlake Capital.
"We are very happy that he has signed and joined our team now,” said Tuchel.
!It is the second signing of the summer. He is the second signing of the summer. He is a strong player, strong personality and big experience. Overall a perfect addition to the team."
Koulibaly and potentially Kounde will replace Antonio Rudiger and Andreas Christensen, who joined Real Madrid and Barcelona respectively after their contracts expired.
Paris Saint-Germain’s Presnel Kimpembe is also a target with Tuchel considering a third defensive swoop amid doubt about the future of Cesar Azpilicueta.
The 32-year-old has agreed a bumper contract with Barcelona, although Chelsea’s valuation of around £7m for their club captain could complicate matters.
Tuchel’s side play their first pre-season friendly against Club America on Saturday in Las Vegas before facing Charlotte FC in Charlotte and Arsenal in Orlando while on their US tour.