Who is West Ham's new record signing?
Diop made his Ligue 1 debut for Toulouse, his hometown club, in November 2015 - a 2-0 win over Nice - before scoring his first goal in a 3-0 victory at Troyes in his second French top-flight appearance.
He skippered the side for the first time in April 2017 before being appointed club captain in October of that year, leading the side to the quarter-finals of the French Cup.
He made 34 Ligue 1 appearances last season as Toulouse struggled, only avoiding relegation with a 4-0 play-off win over Ajaccio in May.
At international level, Diop scored in the 4-0 final win over Italy as France won the European Under-19 Championship in 2016.
"Issa is a player with fantastic potential who has already shown great quality at such a young age," said West Ham director of football Mario Husillos.
"We believe he is now ready to make the next step in his career by moving to the Premier League."
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