The Premier League side have paid a significant fee of more than £25.5 million.
Advertisement
Advertisement
The 18-year-old becomes the first signing for new Hammers manager Julen Lopetegui.
"I’m very happy to be signing with West Ham United, a club that trusted in my work," he told the club's website.
The teenager added compliments to manager Lopetegui, admitting he could learn a lot from the Spaniard.
He went on to say: "The Premier League is the best league in the world.
“I believe I’ll improve a lot here with the entire staff at West Ham, which is crucial for my development."