West Ham have confirmed the club-record signing of France U21 centre-back Issa Diop from Toulouse on a five-year contract.
Sky Sports News understands that he joins for a fee of £21.9m, having successfully passed a medical and finalised personal terms on Monday and Tuesday.
That fee is more than the £20m they paid Stoke for Marco Arnautovic last summer but, if add-ons are reached, that transfer could eventually reach a new Hammers record of £25m.
Diop flew into London on Monday after the deal was agreed between the clubs late last week, and he becomes new manager Manuel Pellegrini's second summer signing after right-back Ryan Fredericks joined from Fulham.
Diop said: "I am very happy to sign for such a historic English club, and I hope to show my best here.
"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."
West Ham's new director of football Mario Husillos said: "Issa is a player with fantastic potential who has already shown great quality at such a young age.
"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."
Monaco, Marseille, RB Leipzig and Sevilla have all been interested in signing the promising young defender.
He made 31 appearances as Toulouse narrowly avoided relegation last season, taking his tally to just under 100 for the club, and has represented France at every youth level from U16 to U21, for whom he has five caps.