Chelsea have signed goalkeeper Edouard Mendy to take their summer transfer window spending towards Stg220 million ($A399m), putting the future starting position of Kepa Arrizabalaga in doubt.
Financial details of Mendy's transfer were not disclosed but British media reported a deal of about Stg22m ($A40m) for the 28-year-old who had three years left on his Rennes contract.
"I am so excited to be joining Chelsea" Mendy told the Blues' website after signing a five-year contract.
"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's decision to boost their goalkeeping options follows the dip in form since last season of Spaniard Arrizabalaga, with his latest error coming in their 2-0 defeat by champions Liverpool on Sunday.
Kepa became the world's most expensive goalkeeper when he was signed by Chelsea two years ago for Stg72m ($A130m) but his errors in games have become increasingly costly.
Mendy played 33 games in all competitions for Rennes last season as they finished third in the French top flight to qualify for the Champions League.
France-born Mendy chose to represent Senegal - the country where his mother was born - at international level and has eight caps.
He becomes the second goalkeeper to make the switch from Rennes to Chelsea with Petr Cech making the move in 2004.
"With Mendy coming in, it's more competition so every reaction will have to be positive around that for Kepa and for Mendy to push to try and play in this team," Lampard said on Tuesday.
"We know the standards that are required of a goalkepeer in this team and we have to push for that."
Mendy's signing follows the arrivals of fullback Ben Chilwell, striker Timo Werner and wingers Hakim Ziyech and Kai Havertz.
The London club also signed defenders Thiago Silva and Malang Sarr on free transfers.
Chelsea, who finished fourth last term to qualify for the Champions League in manager Lampard's first campaign in charge, face West Bromwich Albion in the Premier League on Saturday.
Australian Associated Press