Cameroon defender Nicolas Nkoulou has moved to Marseille from relegated French rivals Monaco on a four-year deal.
The 21-year-old centre back reportedly cost around US$5 million.
As part of the deal, Marseille will loan their Togolese international defender Senah Mango to Monaco with an option to buy.
Nkoulou will join forces with Stephane Mbia at Marseille, who partners him in central defence for Cameroon.
"It's great to meet up again with a brother!" said Nkoulou, who was also the subject of a reported approach from Lyon.
Nkoulou will compete for a place alongside Mbia with Senegal international Souleymane Diawara but said he had no qualms about the competition for a starting berth at France's most popular club.
"Competition is a part of our job," he said.
"I'm a competitor. It's up to me to be ready mentally."
Marseille coach Didier Deschamps heralded Nkoulou's qualities, which he said had been "demonstrated very early on at Monaco, as well as for the national team," and said he had "the potential" to be a first-team player.
Marseille, French champions in 2010, are revamping their squad after losing out to Lille in the Ligue 1 title race last season.
As well as Mbia and Diawara, the French side have Ghana's Ayew brothers, Jordan and Andre, and Burkina Faso's Charles Kabore on their books.