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.
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.