MONACO’s Badiashile, 21, has spent five seasons in Ligue 1 and made 135 league appearances for Monaco.
The central defender made his senior international debut in September against Austria and has two caps.
“I’m very happy to be here at Chelsea. I can’t wait to see the fans and start playing in the best league in the world,” said Badiashile.
Badiashile joined Monaco in 2016 and made his Ligue 1 debut against Paris St-Germain in November 2018 when Thierry Henry was manager.
He has featured in the Champions League and Europa League for the French club and has scored six goals and registered three assists.
“Benoit’s growth over the last two-and-a-half seasons from a young player struggling to fully bring his talent to bear is testament to his and the staff’s attitudes in adopting a higher professional standard,” said Monaco sporting director Paul Mitchell.
“Signing with one of Europe’s biggest clubs is the fruit of the work being accomplished within our academy and first team and once again highlights Monaco’s ability to develop young players to the highest level.”
Blues 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.”
Graham Potter’s side are currently without defenders Wesley Fofana, Reece James and Ben Chilwell through injury.