Malang Sarr will join Monaco on a season-long loan with an option to purchase.
After reaching the end of his contract with Nice, the French defender signed a five-year deal with the London club.
He played in 21 games for the club last season after spending time at Porto.
Last season, Monaco finished third in France and reached the last-16 of the Europa League.
"I am very happy to join Monaco and to be in the same league as before, which I know well."
I would like to express my gratitude to the club. I am looking forward to getting to know my new team-mates and trying to achieve a lot together.
He is the fourth summer signing for Monaco, following the arrivals of Breel Embolo and Thomas Didillon.