Diop, a France under-21 international, is understood to be ready to sign a five-year deal worth £65,000 per week. The Hammers were in competition with Everton and Southampton for the centre-back, but the London club moved swiftly on Tuesday afternoon to strike a deal for personal terms.
Toulouse narrowly avoided relegation to Ligue 2 at the end of last season, beating Ajaccio 4 – 0 on aggregate in the relegation play-off.
Captain of Toulouse last season, the 21-year-old central defender has 85 appearances for the club in his three first-team seasons.
It’s part of Manuel Pellegrini’s restructuring of last season’s porous Hammers defence. Other targets include Swansea duo of Poland international goalkeeper Lukasz Fabianski, 33, and central defender Alfie Mawson, 24.
West Ham have agreed personal terms with Fabianski but remain in discussions with the Swans over the fee. Mawson is some way from becoming a West Ham player with personal terms not yet agreed.
With a significant level of investment at the back, the Hammers are intent on avoiding a repeat of the relegation battle they found themselves ensnared in last season.