France U21 international Magassa, 21, has joined for a fee of about £17.3m.
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"The Premier League is the best league in the world and I'm going to do everything I can to show that I'm up to the task," said Magassa, who can play in a more advanced midfield role and at centre-back.
“I'm very happy to be here. West Ham is a very big Club, it has a long-standing history in England, with many supporters and a very big stadium. It wasn’t complicated to say ‘yes’.
“It's a dream to play for West Ham in the Premier League. It's the best league in the world.
“It gives me great pride to play for this Club. I’m really motivated, I hope to bring freshness and youth and I'm going to give my all on the pitch.”