The France international arrived in London earlier this week to finalise personal terms and undergo a medical, however, paving the way for a transfer reportedly worth around £40m – the second-highest fee ever paid by the Blues to sign a player.
The 22-year-old is the latest star name to depart AS Monaco, trying his luck in English football for the first time in 2017-18 following spells with the Monegasques and Rennes.
Speaking to his new side’s official website, the recently-capped France international said: “I am very happy to be here and to join this great team.
“I have grown up watching Chelsea. Signing was the natural thing for me because it is a club I loved very much in my childhood. I am now looking forward to evolving under this fantastic coach and alongside so many great teammates.”
Bakayoko, who will wear the number 14 shirt next season, becomes Chelsea’s second big arrival of the summer following the signing of Antonio Rudiger from Roma earlier this week.