Frank Lampard has grown impatient with club-record signing Kepa Arrizabalaga, who has consistently made high profile errors during his spell at Stamford Bridge.
Mendy told the club's website: "I am so excited to be joining Chelsea. It’s a dream for me to be a part of this exciting squad and to work with Frank Lampard and all of his coaching staff. I look forward to meeting my team-mates and can’t wait to get started."
Chelsea director Marina Granovskaia added: "As soon as Petr Cech and our technical team identified Edouard as the most suitable goalkeeper to complement our existing group, there was only one player we wanted to bring in.
"Edouard arrives following a season of real success with Rennes, he is ambitious for more, and we welcome him to our club."
A whole host of stoppers have been linked with a move to Stamford Bridge, including Dean Henderson and Mike Maignan, but Chelsea have ultimately chosen to bring in 28-year-old Mendy for a fee worth in the region of £20m.
A busy summer for the Blues has seen them break the Premier League record for money spent in a summer transfer window, with Mendy becoming their seventh addition of the window after signings of Timo Werner, Hakim Ziyech, Kai Havertz, Ben Chilwell, Thiago Silva and Malang Sarr.
Source : 90min