West Ham have signed French defender Issa Diop from Ligue 1 side Toulouse for a club-record transfer fee, the Premier League side announced on Tuesday.
Financial terms of the deal were not disclosed, but reports suggested West Ham paid around £22 million for Diop, eclipsing their previous record signing of forward Marko Arnautovic.
"I am very happy to sign for such a historic, English club, and I hope to show my best here," Diop, who signed a five-year contract, said in a statement.
"I am a very ambitious player and West Ham is a very ambitious club, as we've seen with the appointment of the new coach. I hope to help the team win lots of games and make the fans happy."
Diop made 34 league appearances as Toulouse narrowly avoided relegation last season and has been capped five times by France at Under-21 level.
"Issa is a player with fantastic potential who has already shown great quality at such a young age," West Ham director of football Mario Husillos said.
"It is a measure of his character and maturity that he was named captain of Toulouse at just 20 years of age, having already established himself in their team as a teenager."
Diop becomes new West Ham boss Manuel Pellegrini's second signing of the close season after the arrival of English defender Ryan Fredericks from Fulham on a four-year deal.