Abdou Diallo has become Borussia Dortmund's second-most expensive signing after his arrival from Mainz on a five-year contract was confirmed.
Dortmund are understood to have shelled out €28million to snare the centre-back, eclipsing their outlay for Henrikh Mkhitaryan but falling short of the €30m spent on Andre Schurrle in 2016.
Diallo impressed in his solitary season for Mainz, making 30 appearances in all competitions following his arrival from Monaco for a reported fee of just €5m.
"After getting to know the Bundesliga at Mainz, I am looking forward to the challenge of being able to prove myself at a club as big as Borussia Dortmund," the 22-year-old said.
"Together with my team-mates, I want to justify the trust placed in me and do my part to ensure that BVB achieves its goals in all competitions.
"Now I cannot wait to start preparing for the new season under Lucien Favre."
Diallo made five appearances in Monaco's triumphant 2016-17 Ligue 1 campaign and has captained France Under-21s.
Dortmund sporting director Michael Zorc said: "Abdou Diallo brings a great deal of what we expect from a defender.
"He's a modern, strong and very intelligent centre-back, but he can also play on either side of the defence or be used in defensive midfield.
"The combination of his great potential and his versatility made Abdou so interesting for us."
Diallo could prove to be a replacement for Greece international Sokratis Papastathopoulos, who has been heavily linked with a switch to Arsenal.
