Bakayoko has failed to reach the heights that made Chelsea fork out £40m for his services from Monaco in the summer of 2017.
Despite making 43 appearances, scoring three goals and helping Chelsea win the FA Cup under Antonio Conte in 2017/2018, Bakayoko could not convince the Stamford Bridge faithful with his performances.
Bakayoko spent last season on loan at AC Milan, helping the club to a fifth place finish but the Rossonerri opted against making the deal permanent at the end of the campaign.
The 25-year-old, who has one cap for the French national team, has failed to make any of Frank Lampard’s matchday squads so far this season and will now join Chelsea’s legion of loanees at his former side.
"I am very happy to come back to AS Monaco, a club with which I keep fantastic memories," he told Monaco's website.
"Since 2017, I had the opportunity to have two very good experiences with Chelsea and [AC] Milan.
"I hope to be able to integrate as quickly as possible in this collective, in an environment that I already know, and especially with the desire to bring to the group what I know how to make a good season."
Bakayoko helped Monaco win the French Ligue 1 title in the 2016/2017 season, and they also made the semi-finals of the UEFA Champions League that same year.
He will be reunited with former Chelsea teammate Cesc Fabregas, who joined the French side earlier this year after four-and-a-half seasons at Stamford Bridge.
