The 22-year-old centre-back has signed a seven-and-a-half year contract with Chelsea and arrives on a reported €38m fee.
Badiashile becomes Chelsea’s second signing of the January transfer window, joining the 20-year-old striker David Datro Fofana, and it follows a record-breaking summer of business which also saw the Blues invest heavily in defenders Wesley Fofana, Kalidou Koulibaly and Marc Cucurella.
Chelsea had been keen to sign a left-sided centre-back this window, having also been linked with RB Leipzig and Croatia defender Josko Gvardiol.
Chairman Todd Boehly said: “We’re delighted to welcome Benoit to Chelsea. He’s an excellent young player who has quickly established a strong reputation for himself and has a huge amount of experience for his young age. We are sure he will form an important part of our team in the years ahead.”
Badiashile, who made his senior debut for France in September having represented his national team at every youth level from the U-16 stage, made over 100 appearances for Monaco across five seasons at the French club.
“I’m very happy to be here at Chelsea,” he said. “I’m so excited to start playing for this club. I can’t wait to see the fans and start playing in the best league in the world.”
Chelsea are 10th in the Premier League table following a difficult spell under manager Graham Potter, who replaced the sacked Thomas Tuchel earlier this season.
But Boehly is taking a longer-term view and wants to help Potter build his squad. Chelsea are also interested in the Shakhtar Donetsk and Ukraine winger Mykhailo Mudryk, and are prepared to pay up to £88m to beat rivals Arsenal to his signature.