The London-based club have been working on getting a deal over the line for the 28-year-old after identifying him as an ideal goalkeeping candidate to compete with Kepa Arrizabalaga.
Chelsea had agreed terms with Mendy, who is keen on a move to Stamford Bridge, but were in talks with Rennes over a fee.
Now Frank Lampard’s side have found a breakthrough and have struck a deal with Rennes over a deal for the goalkeeper.
While the two clubs are understood to have shaken hands on a deal, it remains to be seen what the final fee is.
Mendy is now expected to travel to London to complete the final procedures before putting pen to paper on a contract with Chelsea.
The goalkeeper is claimed to have agreed to a five-year contract with the Blues.
The arrival of Mendy will see him join the likes of Timo Werner, Hakim Ziyech and Thiago Silva in becoming the latest additions to Chelsea in the current transfer window.